MODULE 5. RAMP CONTROL
Manual TABLE OF CONTENTS
Module 5. TABLE OF CONTENTS
MODULE 5. RAMP CONTROL
TABLE OF CONTENTS
5.1 INTRODUCTION
............................................
DEFINITION OF RAMP CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APPLICATION OF RAMP CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Entrance Ramp Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Entrance Ramp Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exit Ramp Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Systemwide Ramp Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RELATION TO OTHER FREEWAY MANAGEMENT FUNCTIONS . . . . . . . . . .
Surveillance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vehicle Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Closed-Circuit Television . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Environmental Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HOV Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Information Dissemination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Control Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BENEFITS OF RAMP CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Improved System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Improved Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reduced Vehicle Operating Expense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Means for Positive Freeway Traffic Control/Management . . . . . . . . . . . . . . . . . . .
Reduction in Vehicle Emissions and Fossil Fuel Consumption . . . . . . . . . . . . . . . .
Coordination With Other Corridor Management Elements . . . . . . . . . . . . . . . . . .
Promotion of Multimodal Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MODULE OBJECTIVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MODULE SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 DECISION PROCESS
5-5
5-5
5-6
5-6
5-6
5-6
5-6
5-6
5-7
5-7
5-7
5-7
5-7
5-7
5-7
5-7
5-8
5-8
5-8
5-8
5-8
5-9
5-9
5-9
5-9
5-9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
PROBLEM IDENTIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
Level of Service / Capacity Deficiency / Bottlenecks . . . . . . . . . . . . . . . . . . . . . . 5-10
Vehicle Crash Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
Inventory of Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11
Freeway System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11
Existing Freeway Management Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11
Existing Ramp Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Other Relevant Field Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Inventory of Traffic Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Traffic Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Traffic Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Other Traffic Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Temporal Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13
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Ramp Geometric Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cross Streets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Service Roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary of Problem Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDENTIFICATION OF PARTNERS AND CONSENSUS BUILDING . . . . . . . . .
Relation to Other Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
City/County Traffic Operations Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . .
Enforcement Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Emergency Management Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Public Transportation Agencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relationship to Elected Official / Political Environment . . . . . . . . . . . . . . . . . . .
Importance of Enforcement / Judicial System . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relationship With Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ESTABLISHING GOALS AND OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . .
ESTABLISH PERFORMANCE CRITERIA / MEASURES OF
EFFECTIVENESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DEFINE FUNCTIONAL REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DEFINE FUNCTIONAL RELATIONSHIPS, DATA REQUIREMENTS, AND
INFORMATION FLOWS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-13
5-14
5-14
5-14
5-14
5-14
5-14
5-14
5-15
5-15
5-15
5-15
5-15
5-16
5.3 TECHNIQUES AND TECHNOLOGIES . . . . . . . . . . . . . . . . . . .
5-18
5-18
5-18
5-19
5-19
5-20
5-20
5-20
5-22
5-22
5-23
5-24
5-24
5-24
5-24
5-25
5-26
5-26
5-26
5-28
5-28
5-29
5-30
5-31
5-32
5-32
ENTRANCE RAMP CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ramp Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ramp Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Metering Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Congestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pretimed Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Single-entry metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Platoon metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tandem Metering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Two-abreast Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Traffic-Responsive Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fundamental Traffic Flow Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Demand-Capacity Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Occupancy Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gap-Acceptance Merge Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-2
5-16
5-17
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System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Ramp Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Pretimed Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Example 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Practical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Example 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Traffic-Responsive Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Metering Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
System vs. Independent Ramp Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Controller Interconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Incremental Benefits of Various Levels of Control . . . . . . . . . . . . . . . . . . . . . . .
Variations in the Ratio of Mainline to Entrance Ramp Demand . . . . . . . . . .
Variations in Overall Traffic Demand Pattern . . . . . . . . . . . . . . . . . . . . . . .
Mainline Capacity Reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EXIT RAMP CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EMERGING TECHNOLOGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-33
5-35
5-35
5-36
5-38
5-38
5-40
5-41
5-43
5-43
5-43
5-43
5-43
5-44
5-44
5-44
5-44
5-45
5-45
5.4 LESSONS LEARNED
5-46
5-47
5-47
5-47
5-48
5-48
5-49
5-50
5-51
5-51
5.5 EXAMPLES IN RAMP CONTROL
5-52
5-52
5-52
5-52
5-53
5-54
5-55
5-56
5-56
5-56
5-57
5-57
......................................
IMPLEMENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Public Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Media Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Implementation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Implementation Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OPERATIONS AND MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DIVERSION OF TRAFFIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ENFORCEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EQUITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.......................
HISTORY OF RAMP CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ENTRANCE RAMP METERING CASE STUDIES . . . . . . . . . . . . . . . . . . . . . . . .
Portland, Oregon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Minneapolis/St. Paul, Minnesota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seattle, Washington . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Denver, Colorado . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detroit, Michigan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Austin, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Long Island, New York . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
San Diego, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SUMMARY OF RAMP METERING BENEFITS . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 REFERENCES
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-59
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MODULE 5. RAMP CONTROL
Figure 5-1. Ramp Meter with HOV By-Pass in Minneapolis, MN.
individual metered ramps numbering over
2300.(1)
5.1 INTRODUCTION
The geometric design of a freeway ramp
(width, curvature, vertical alignment, etc.)
can have a positive or negative influence on
both the operation of the ramp itself and on
the freeway at, or upstream of, the merge
point. Freeway design standards generally
address those considerations. Ramp control,
on the other hand, seeks to regulate the flow
of vehicles at freeway ramps in order to
achieve some operational goal such as
balancing demand and capacity or enhancing
safety. Other than freeway-to-freeway
interchanges, freeway ramps represent the
only opportunity for motor vehicles to
legally enter or leave a freeway facility and,
therefore, the only point at which positive
control can be exercised. Freeway ramp
control systems have been in operation at
various locations throughout the country
since the early sixties. It is estimated that
ramp control systems are operated in over
20 geographical areas at present, with
Most ramp control systems have been
proven to be successful in terms of reduced
delay and travel time (and the concomitant
reductions in fuel consumption and vehicle
pollutants, and in accident reduction.) They
are more effective when they are part of an
integrated transportation management plan
that incorporates other systems as described
in other modules of this document.
Deployment of ramp control systems has
been somewhat limited due to some public
resistance to being stopped on a freeway
ramp for no readily apparent reason,
although the ramp metering rate may reflect
a downstream bottleneck such as an incident.
DEFINITION OF RAMP CONTROL
Freeway ramp control is the application of
control devices such as traffic signals,
signing, and gates to regulate the number of
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vehicles entering or leaving the freeway, in
order to achieve some operational objective.
Typically, the objective will be to balance
demand and capacity of the freeway in order
to maintain optimum freeway operation and
prevent operational breakdowns. Ramp
metering may also be applied for safety
considerations where certain geometric
inadequacies or other constraints exist.
necessary to physically close the ramp with
automatic gates or manually placed barriers.
Obviously, this extreme case may cause
negative public reactions and should be
applied only after considerable planning and
a public information program.
Exit Ramp Closure
Metering of exit ramps is obviously not
appropriate but closure with automatic gates
or manually placed barriers, with adequate
freeway warning signs, may be used to
accomplish certain operational objectives.
For example, if the exit ramp terminus at the
cross street has such inadequate capacity
that exit ramps queue onto the freeway, the
ramp may be closed to encourage drivers to
exit upstream or downstream where more
capacity is available.
APPLICATION OF RAMP CONTROL
The primary application of ramp control,
commonly known as ramp metering, has
been on freeway entrance ramps. However,
ramp control has been applied in other
situations as well.
Entrance Ramp Metering
Metering on entrance ramps involves
determination of a metering rate (typically “4
to 15 vehicles per minute” are minimum and
maximum rates for single lane metering)
according to some criteria such as measured
freeway flow rates, speeds, or occupancies
upstream and downstream of the entrance
ramp. The rates may be fixed (pre-timed)
for certain periods, based on historical data,
or may be variable minute-by-minute (traffic
responsive) based on measured traffic
parameters. The entry of vehicles at that
rate is regulated by one or more traffic
signals beside the ramp at driver’s-eye
height. Vehicle sensors may be located at
points along the ramp to signal the blockage
of the merge area or backing of the ramp
queue into a cross street.
Systemwide Ramp Control
Although individual ramps may be metered
or closed for specific reasons, ramp control
is most effective when ramps are metered in
an integrated system manner. Individual
metering rates are determined by conditions
over a larger portion of the freeway, not just
in the immediate area of the ramp. Although
local controllers may suffice in individual
ramp metering as described above, systemwide control requires a central or distributed
control system master with control
algorithms and interconnection by some
communications media.
RELATION TO OTHER FREEWAY
MANAGEMENT FUNCTIONS
Entrance Ramp Closure
Ramp control is closely related to other
infrastructure elements in a freeway
management system. The widespread,
widely embraced Intelligent Transportation
Systems (ITS) movement has further
emphasized the benefits of integrated system
elements. Other modules in this handbook
Typically lower metering rates (say 2 to 4
vehicles per minute) over a sustained period
of time are not acceptable to drivers, and
they will tend to disregard the signal. In the
extreme case where the metering rates must
be sustained at lower levels, it may be
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describe specific subsystems of a freeway
management system.
The following
paragraphs briefly describe the relationship
of those elements to freeway ramp control.
Environmental Sensors
Due to grades on ramps it is often necessary
to adjust ramp metering rates or terminate
operation during extreme weather conditions
such as icy or extremely wet roadway
surfaces. Environmental sensors will give
early warning when such conditions exist.
Surveillance
The surveillance subsystem includes various
techniques for determination of freeway and
ramp operating conditions that may have an
influence on metering rates or operational
overrides.
Specific information on
surveillance technology can be found in
Module 3 of this handbook. The paragraphs
below provide a description of the types of
surveillance used in conjunction with ramp
control.
HOV Treatments
Preferential treatment of high-occupancy
vehicles at entrance ramps has been used
successfully in several locations on entrance
ramps. These systems have primarily
involved a separate lane to bypass the ramp
signal, and single occupant vehicle queue.
Vehicle Detection
Information Dissemination
Vehicle sensors located on the freeway can
serve multiple purposes if located correctly
during the design and construction phase.
Detectors located in the freeway lanes
generally have the purpose of input to
incident detection algorithms and for system
operation evaluation. Freeway detectors can
also be used as input data in determining
metering rates in traffic responsive
operations. Counting detectors located on
entrance and exit ramps serve as input and
output data in defining a closed system
operation for estimating average delay in the
system.
Notification of travelers of ramp closures
can be effected by either pre-trip information
dissemination devices such as kiosks, Web
site, and Community Access Television
(CATV), or by on-road devices such as
variable message signs or highway advisory
radio. Other operational changes in ramp
operations that may be of interest or
assistance to travelers may also be
communicated.
Communication
Unless the controlled ramps are isolated and
operate in a nonsystem mode, the
communication
subsystem
must
accommodate for the control, detection, and
signal hardware.
Closed-Circuit Television
Closed-circuit television (CCTV) are used to
detect and verify incidents in the overall
surveillance subsystem. Cameras can also be
used to fine tune and monitor operation of
individual metered ramps, precluding the
necessity for on-site field observation.
Control Center
While ramp control systems generally have
the capability to operate in an isolated
manner without supervision from a central or
distributed master, most are interfaced to a
central management system through the
communication system.
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BENEFITS OF RAMP CONTROL
Improved Safety
Positive benefits of ramp control have been
documented widely and can be found in the
general literature.(1) Benefits have been most
commonly reported in typical measurable
traffic operations parameters such as
reduced delay and travel time, increased
throughput and operating speeds, and
reduced accident experience. Other benefits
less easily quantified may also accrue from
ramp control. The case studies in the
following section summarize some reported
benefits of ramp control. The following
paragraphs describe typical benefits, both
quantifiable and less easily quantified.
Freeway ramp control can effect decreased
vehicle crash experience on both the ramp
(and merging area) and on the freeway. By
breaking up platoons of vehicles, which may
enter the ramp from discharge at an adjacent
intersection or traffic generator, the
incidence of rear end vehicle crashes is
decreased in the merging area, where
multiple vehicles compete for gaps. Vehicle
crashes on the freeway are also reduced as
the merge becomes smoother, and freeway
drivers in the outside (merging) lane are less
likely to have to brake abruptly or make
lane-change maneuvers. Finally, in systemwide operation the overall freeway is
maintained in a more stable, uniform
operational mode and vehicle crashes
resulting from stop and go operations are
reduced.
Improved System Operation
Freeway traffic operating characteristics that
can be expected to be influenced by ramp
control systems are: speed, travel time, and
delay. Typically, freeway operation has been
described as a series of relationships between
volume, speed, and density (or occupancy).
The general objective of most freeway
management systems is to optimize
throughput while maintaining freeway
operation in the non-congested area of the
curve. By controlling the number of vehicles
entering the freeway based on available
downstream capacity to accommodate
upstream freeway vehicles and entering ramp
vehicles, freeway operation is enhanced. In
another scenario, the objective may be to
maintain some target level of service (as
indicated by speed.) Again, by controlling
the rate at which vehicles are metered onto
the freeway, a target operating condition is
maintained. Improvements on the freeway
must be weighed against ramp delays and
travel times which may be increased for
travelers who choose to divert to other
facilities.
Reduced Vehicle Operating Expense
Improved system operation has the direct
and quantifiable result of reduced vehicle
operating expense. Reductions in the
number of stops and speed changes translate
to related reductions in vehicle operating
expense. The most significant savings are
related to the reduction of vehicle crashes.
Means for Positive Freeway Traffic
Control/Management
There are few opportunities to actively
“control” freeway traffic on a routine basis.
Obviously, police officers working freeway
incidents control freeway traffic, but not on
an everyday basis at the same location.
Passive control, such as suggestions or
advisories via pre-trip planning information
sources or en route signing, may either be
followed or ignored. Ramp control offers a
means to regulate or control freeway bound
vehicles.
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Reduction in Vehicle Emissions and
Fossil Fuel Consumption
Promotion of Multimodal Operation
By giving preferential treatment to High
Occupancy Vehicles at entrance ramps, the
ramp control subsystem can promote travel
mode shifts and reduction of single
occupancy vehicles.
The direct correlation between improved
traffic operations and the reduction of fuel
consumption and vehicle emissions is wellknown. Reductions in delay and numbers of
stops, together with the maintaining of more
uniform speeds as described previously will,
in virtually every situation, result in a similar
reduction in fuel consumption and vehicle
pollutants. An exception might be where
speeds are in higher ranges than is typically
experienced during peak periods on
metropolitan freeways.
MODULE OBJECTIVE
The objective of this ramp control module is
to provide insights into and guidelines on the
issues associated with planning, designing,
constructing, operating, and maintaining a
ramp control subsystem in a freeway
management system. This module also gives
guidance to planners, designers, managers,
and operators in public relations aspects of
freeway ramp control.
Coordination With Other Corridor
Management Elements
Intelligent Transportation Systems (ITS)
defines certain core infrastructure elements
known collectively as an intelligent
transportation
infrastructure.
The
importance of the interrelationship of the
various subsystems applies to ramp control
as a subsystem of Advanced Transportation
Management Systems (ATMS) as well.
Examples include the following:
C
Ramp metering systems should be
coordinated with surface street traffic
signals to account for spill back of ramp
queues.
C
Information on ramp closures may be
communicated
by
off-freeway
information devices.
C
High Occupancy Vehicle programs may
involve special treatment of HOV at
entrance ramps.
C
Special ramp operating procedures may
be instituted during incident conditions.
MODULE SCOPE
The scope of this ramp control module is
intended to include general guidelines as well
as serving as a guide to references and other
documentation that may be of benefit to
planners, designers, and operators of
freeway management systems. It is not
intended to provide detailed design
specifications or other construction
documents. Typical plans, specifications,
and estimates documents can usually be
obtained from agencies already operating
ramp control systems.
5.2 DECISION PROCESS
Freeway ramp control is one of the few
direct means of controlling access to the
freeway main lanes. Indirect control would
include such methods as encouraging
diversion to other facilities, or mode changes
through communications with travelers prior
to their trips or en route. However, direct
limiting of access through ramp control can
be effective and accepted by the driver only
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if it is applied in those circumstances where
traffic characteristics, demand patterns, and
infrastructure are conducive to the
technique.
Level of Service / Capacity Deficiency /
Bottlenecks
The Highway Capacity Manual provides
definitive guidance in determining qualitative
and quantitative pictures of freeway
operations. Capacity deficiencies, or freeway
bottlenecks, will be a function of traffic
demand and characteristics as well as the
geometrics and other design features of the
roadway itself. It will be necessary to
pinpoint where such deficiencies exist, and
the contributing factors.
Subsequent
sections of this module provide a detailed
listing of data required for analysis of
capacity and level of service in relation to
ramp control, as well as other analyses.
Module 2 provides more guidelines for
capacity and level of service analyses.
PROBLEM IDENTIFICATION
Traffic engineers and other professionals will
no doubt have an intuitive feel for where
freeway operational deficiencies exist in a
congested freeway environment. However,
in order to address potential solutions to
alleviate such problems, it will be necessary
to quantify deficiencies in both time and
space, i.e. during what portions of the day,
and at which locations within the freeway
system, are such deficiencies present. It is
important to document the freeway
operations from a traffic characteristics and
infrastructure aspect in order to identify and
define the problem as well as to provide a
basis for measurement of effectiveness and
to monitor for future changes. Several
techniques may be used to illustrate a
systemwide picture of freeway traffic
characteristics, design features, capacity
deficiencies, vehicle crash experience, and
other features of interest, including the
following:
C
Capacity and Level of Service (LOS) should
be determined for existing traffic
characteristics and infrastructure as well as
those parameters for future conditions in
some horizon or build-out year. Planned
additions to the freeway section under
consideration, or to alternative routes or
modes, may either obviate the need for ramp
control or influence its implementation
schedule.
Schematic maps, color coded or
otherwise delineated to show various
levels of operation, other traffic
characteristics, and crash experience at
various periods of the day.
Vehicle Crash Experience
C
Schematic maps, color coded or
otherwise delineated to show various
infrastructure characteristics.
The occurrence of vehicle crashes on
freeways may be attributed to a variety of
factors, some of which may not be
correctable by ramp control techniques.
Those types of accidents most likely to be
alleviated by ramp control include:
C
Spreadsheet or other tabular format.
C
C
Descriptive write-up.
Rearend crashes on freeway main lanes
due to over-capacity operation
(bottleneck conditions).
C
A combination of the above items.
C
Lane change crashes on freeway lanes
due to over-capacity merging conditions.
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C
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Lane change crashes on freeway lanes
due to inadequate sight distance or to
other geometric deficiencies in the merge
area.
C
Run-off-the-road crashes caused by
drivers avoiding shock waves.
C
Rearend crashes on the entrance ramp
due to queuing in the merge area.
Type of ramp design (loop, linear)
should be noted.
C
Frontage Roads. Presence of frontage
roads and their lane configurations
should be determined and tabulated.
C
Interface to Crossing Freeways.
Freeways will generally be interfaced or
connected via a freeway interchange.
The proximity of another freeway’s
connections to the entrance ramps being
considered for ramp control should be
noted to determine if any special
measures are needed.
C
Interface to Crossing Arterials. The
relationship of entrance ramp metering
to an upstream cross street is critical. If
not properly considered, queuing from
the ramp signal into the cross street can
cause concerns to the agency responsible
for arterial street operation, as well as
public resentment. Type of crossing
roadway, traffic control, mix of traffic,
ramp storage area, and other factors
should be noted for each ramp.
Crash records may be summarized by section
of freeway, location, time of day, and type of
crash to determine if ramp control has the
potential to reduce collision experience.
Inventory of Infrastructure
Except in the case of an isolated, single
entrance ramp location, a ramp control
system is generally a subsystem of a
comprehensive freeway management system.
Much of the infrastructure data required for
problem identification will likely be available.
The following types of data should be
assembled for the freeway system under
consideration.
Freeway System
Existing Freeway Management Systems
C
Lane Configuration. Number and
types of freeway lanes (through,
weaving, acceleration, deceleration)
should be determined and tabulated
and/or graphically displayed.
C
Ramp Locations. Entrance and exit
ramps should be located, with link
distances between ramps determined.
C
Geometrics. Typical geometrics such as
freeway lane and entrance ramp width,
vertical and horizontal alignment, ramp
length, ramp storage capacity, merging
area, and sight distance restrictions
should be determined and tabulated.
Normally there will be only one agency
responsible for freeway operations in a
particular geographic area, but there are
some situations where more than one agency
may be involved. For example, a dense
metropolitan area may extend into two
States or a tollway operated by a toll
authority may interface to a state-operated
freeway. As part of the inventory, the
existence of such systems should be
confirmed and documented to include the
following:
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C
Participating agencies.
C
Type and location of control center
facility.
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C
Type of control system
distributed, hybrid, local).
Module 5. TABLE OF CONTENTS
(central,
C
Surveillance and detection.
C
Information dissemination (pre-trip, en
route).
C
Communication system (medium, leased,
or owned).
Inventory of Traffic Characteristics
Certain traffic and flow characteristics will
influence the potential success and the
design of freeway ramp control systems.
Typical traffic characteristics are listed
below. Module 2 provides a more detailed
description of individual traffic parameters.
Traffic Composition
Existing Ramp Control Systems
In lieu of a full blown freeway management
system, some entrance ramps may be
metered in an isolated manner with a local,
non-system controller. Inventory should
include the following:
The composition of the traffic stream on the
freeway main lanes and the entrance ramp
will influence both the type of control and
the design of the system. A determination of
the percentage of passenger vehicles,
commercial vehicles, and transit vehicles
should be made for peak periods.
C
Responsible agency.
Traffic Flow
C
Type of controller.
C
Surveillance and detection.
C
Communication system.
Traffic volumes and traffic flow rates during
peak periods will be required for capacity
and level of service determinations to define
the operating conditions and problem
locations that might be addressed by ramp
control techniques. Traffic flow data will
also be used in determining metering rates
and periods of operations. Traffic flow data
requirements will include the following:
Other Relevant Field Systems
Other relevant field operational systems that
may have an effect on freeway operation
should also be identified. Such systems
would include the following:
C
High occupancy vehicle lanes or ramps.
C
Incident management teams.
C
Traffic volumes and flow rates, generally
by 15-minute periods, on freeway lanes
and entrance and exit ramps.
C
Distribution of freeway vehicles by lane.
C
Traffic volumes and flow rates on
adjacent service roads.
Traffic volumes and flow rates on cross
streets served by the freeway ramps.
C
Accident investigation sites.
C
Courtesy and motorist assistance patrols.
C
C
Hazardous material
restrictions.
Other Traffic Parameters
routing
and
Other typical traffic parameters that may be
of value either in defining operating
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conditions and problem locations or in
developing control strategies include the
following:
C
C
C
C
Temporal Variations
As previously mentioned, it is important that
traffic operations characteristics be collected
and analyzed in incremental time periods so
that ramp control operation schedules can be
developed optimally. Even though the
system may be traffic responsive, it may be
advantageous to operate either on a
predictable schedule or with limited
variations in schedule. Plotting various
parameters by time period in 15-minute
increments will help predefine those
operational periods. Although ramp control
is usually associated with peak periods,
plotting data over a longer period may
indicate other times when ramp metering
may be appropriate.
Lane Occupancy. Defined as the
percentage of time a particular sampling
“spot” on the freeway is occupied, this
parameter may not be economically
measured until such time as a
surveillance system is in place. Its
primary use is in selecting metering
rates, although it can identify operational
problems if reasonably available. It may
be derived from speed and volume data,
which may be more readily available
prior to system implementation. The
reader is referred to Module 2, or the
Highway Capacity Manual, for a
discussion of the relationship of lane
occupancy to freeway level of service.(2)
Ramp Geometric Limitations
Inventory of infrastructure elements and field
observations will provide information to
evaluate the physical viability of individual
ramps to support ramp metering. The
following physical factors should be
considered:
Traffic Density. Defined as the number
of vehicles per lane per mile, traffic
density may be determined with aerial
photos or by freeway input/output
counts. The reader is referred to
Module 2 or the Highway Capacity
Manual for a discussion of the relation
of traffic density to freeway level of
service (LOS).(2)
Speed. Vehicle speeds are another
indicator of freeway LOS and may be
determined by traditional speed
measurement techniques prior to system
installation.
Vehicle Occupancy. As opposed to
lane occupancy, vehicle occupancy is
generally defined as passengers per
vehicle and is usually determined by
manual observation. This parameter may
be useful in determining the viability of
preferential treatment of high occupancy
vehicles (HOV) at entrance ramps.
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C
Ramp Storage. How many vehicles can
reasonably be stored or queued on the
ramp upstream of the metering signal
without interfering with cross street
traffic?
C
Ramp Width. Is there adequate width
for side-by-side metering and/or
preferential HOV bypass lanes?
C
Grade. Are ramp grades restrictive
during adverse weather or for certain
types of heavy vehicles?
C
Merge Area. Does the present design
facilitate a smooth merge?
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will be offset by overall system
improvement, but this is not always apparent
to the driver. Without ramp control, drivers
may experience even more delay on the
freeway than they would have experienced at
the signal. Again, this may not be readily
discernable to the driver.
Cross Streets
Limited vehicle storage for queuing at ramp
signals may adversely affect operation of an
upstream cross street. Presence of such
conditions should be noted so that they can
be considered during design of control
strategies.
In most instances, a State Transportation
Department or Toll Authority will have the
responsibility for operation, but not
necessarily enforcement, of a ramp control
system. It is important for the agency
responsible for operation of the ramp control
system to identify and establish relationships
and communications with all agencies that
may have a role in operation and
enforcement so that they may be brought
into the planning and design process. It is
also important that the benefits of ramp
control, which are realistic and measurable,
be fully explained and that it not be oversold
as adding capacity (such as adding a lane).
It should be characterized as a means to
make maximum use of available capacity by
managing capacity and demand.
Service Roads
As with cross streets, limited vehicle storage
for queuing at ramp signals may adversely
affect operation. The type of cross street
(major arterial, collector, etc.), traffic
demand, presence of signals, and their
operation must be considered.
Summary of Problem Definition
Traffic characteristics and demand, as well
as geometric factors, are important in
evaluating existing and future conditions and
the potential applicability of freeway ramp
control. While not all data items listed
above may be available to the designers and
planners, it is important to collect and
assemble as much relevant data as feasible
for the analysis. Many of the data items
noted above may also be used during the
design of the system and development of the
control strategies and software.
Relation to Other Agencies
City/County Traffic Operations Agencies
Because of the close relationship and
interface between surface street traffic
operation and signalization and access to and
from freeway ramps, it is important to
involve those agencies and build a consensus
for the system at all levels, from the agency
head to the operations engineers to the
control system operators. To the extent
possible, system goals and objectives should
be developed mutually.
IDENTIFICATION OF PARTNERS
AND CONSENSUS BUILDING
Freeway ramp control is the primary method
of managing demand once drivers have
committed to use the freeway for their trip.
It has been proven to be an effective means
of balancing capacity and demand and
reducing delay and vehicle crashes. It can
also be one of the most controversial traffic
control techniques. Delay at ramp signals or
closing of a ramp may be considered too
drastic by some drivers, and even an
infringement on their rights. Such delays
Enforcement Agencies
Depending on State and local ordinances or
interagency agreements, State, local, or
transit police may be responsible for
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enforcement of ramp control devices.
Compliance with ramp control signals is
essential if the system is to operate
efficiently. Enforcement agencies must be
brought into the process early and must
understand the goals and objectives of the
system and the operating philosophies. The
signals must be enforced, but overenforcement can have a detrimental effect on
driver attitude and, in fact, cause
deterioration of operation as drivers are
stopped on the freeway shoulder.
Compliance with the signals must be
established early and monitored to ensure
that an acceptable level is maintained. A
program of public information and police
support is essential.
ramp control are real and measurable in the
overall system, but may not be apparent to
the individual driver who experiences delay
at an entrance ramp or must reroute due to
a ramp closure. Citizen (voter) complaints
can have an adverse effect on the success of
ramp control projects. System planners,
designers, and operators must help those in
office understand the goals, objectives, and
operating characteristics of the system prior
to system turn-on.
Importance of Enforcement / Judicial
System
The importance of enforcement of ramp
control has been previously stated.
Accordingly, enforcement must be supported
by the judicial system. A standard ramp
traffic signal that meets the requirements of
the Manual On Uniform Traffic Control
Devices (MUTCD) is a legally enforceable
device.(3) However, because ramp control
systems are not as familiar as intersection
signals, certain judges may be inclined to
dismiss related citations. It is important to
ensure that the proper laws and ordinances
are in place and that judges to whom appeals
of citations may be taken are informed of the
system goals, objectives, and operating
characteristics prior to system turn-on.
Emergency Management Agencies
Fire, police, medical, hazardous materials,
motorist assistance patrols, and other
agencies responsible for emergency
management on the freeway system should
be aware of the proposed system and be fully
informed as to its operation and benefits.
Any special support required of the
particular agency should be solicited.
Public Transportation Agencies
Public transit agencies that access freeways
via metered ramps, or that exit on ramps
which may be closed during certain periods
of the day, should be also be brought into
the planning and design process at an early
stage. This is particularly important where
preferential treatment of high-occupancy
vehicles such as buses is being considered.
Relationship With Media
Relationship to Elected Official /
Political Environment
Local news media, both print and electronic,
can have a profound effect on the success of
ramp control systems. It is important that a
media relations plan be developed to help
ensure that positive support is secured.
Methods for disseminating information about
ramp control system include brochures, town
meetings, and handouts.
Although a support base and consensus may
be built at the staff and agency level, it is
important to build support with elected
officials as well. As stated above, benefits of
As stated previously, it is important that the
benefits of ramp control, which are realistic
and measurable, be fully explained and that
they not be oversold as adding capacity (as
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C
in the case of adding a lane). It should be
characterized as a means to make maximum
use of available capacity by managing
capacity and demand.
C
ESTABLISHING GOALS AND
OBJECTIVES
Module 2 describes the process of
establishing system goals and objectives.
Goals and objectives of the ramp control
system should complement and not conflict
with overall system goals. In the rare case
of stand-alone ramp control system, the
goals and objectives may differ from those in
an integrated system.
Typical overall system goals and objectives
and how they may be supported by a ramp
control system are listed below.
C
Reduced
Accident
Experience.
Maintaining smoother freeway flow by
metering and improving merge
conditions on the ramp.
C
Maintaining Acceptable Freeway
Level of Service. Metering on entrance
ramps to maximize freeway flow rates
within acceptable ranges.
C
Balancing Demand/Capacity in
Freeway Corridor.
Metering on
entrance ramps to encourage drivers to
shift to other ramps or facilities with
available capacity, or to change trip time.
C
Reduction of Single-Occupancy
Vehicles. Preferential treatment of car
pools on entrance ramps.
C
Reduced Vehicle Delay. Metering on
entrance ramps to limit freeway flow
rates within acceptable ranges.
Incident Management. Closing ramps
upstream of a freeway incident and
increasing metering rates downstream.
Promotion of Multimodal Operation.
Preferential treatment of buses on
entrance ramps.
C
Reduced Noise. Smoother Traffic Flow
Reduces Engine Revving.
C
Reduced Vehicle Operating Costs. A
result of smoother traffic flow and
reduced stops.
C
Reduced Fuel Consumption. A result
of smoother traffic flow and reduced
stops.
C
Reduced Vehicle Emissions. A result
of smoother traffic flow and reduced
stops.
ESTABLISH PERFORMANCE
CRITERIA / MEASURES OF
EFFECTIVENESS
Performance criteria express broad goals in
tangible or measurable terms.
Better
operation is obviously a goal to be strived
for, but is difficult to measure and may have
different meanings for different people.
With the exception of the first goal
(balancing capacity and demand), the goals
and objectives listed above are tangible and
measurable in readily understandable terms
both before and after system turn-on. Level
of service can be calculated, vehicle crash
rates can be tabulated from law enforcement
data bases, speed and delay studies can
determine operating conditions that can be
used to calculate delay, fuel consumption,
and vehicle emissions. Transit records are
available to establish changes in bus
patronage, and field studies can measure
vehicle occupancy.
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design of intelligent transportation
systems. It is not a system design
concept. What it does is define the
framework around which multiple
design
approaches
may
be
developed, each one meeting the
needs of the user, while maintaining
the benefits of a common
architecture. The architecture design
defines functions (e.g., collect data
from freeway and ramp detectors;
and operate and monitor ramp
meter signals) that must be
performed to implement a given user
service, the physical entities or
subsystems where these functions
reside (e.g., detectors on the freeway
and ramp, signals on ramps, and
local controller near the ramp), the
interfaces/information flows between
the physical subsystems and the
communication requirements for the
information flows (e.g., signal
wirelines from the detector to the
local controller and from the
controller to the ramp signal; twoway wideband communication
between the field controller and the
central management site.)
In
addition, it identifies and specifies
the requirements for the standards
needed to support national and
regional interoperability.
DEFINE FUNCTIONAL
REQUIREMENTS
Functional requirements for the ramp control
subsystem are fairly straightforward and are
summarized below:
C
Displays. Signals on the ramp for
vehicle drivers and advance warning
signs.
C
Local Controller. Device to receive
and store vehicle detector information
and operate signals according to internal
logic or according to a central
supervisory system.
C
Vehicle Detectors. Devices to measure
conditions on the freeway and ramp.
C
Control Logic. Programs residing in
the local controller for non-system
operation, or at a central system
processor for system operation.
C
Communications. Leased or owned
communication link between field
location and central management site for
data and control command transmission.
C
Central Control System. Computer,
peripherals, and operator interface
devices.
In all likelihood, the functional relationships,
data requirements, and information flows for
a ramp control system will be dictated by the
design of the broader freeway management
system. However, in the case of an isolated
ramp control system, the architecture will be
more in the realm of typical signal design at
an arterial street intersection. In any event,
an open architecture (one that can be
interfaced with in the future) should be
employed.
DEFINE FUNCTIONAL
RELATIONSHIPS, DATA
REQUIREMENTS, AND
INFORMATION FLOWS
The June 1996 ITS Architecture Executive
Summary states (italics indicate adaptation
to ramp control systems):
The National ITS Architecture
provides a common structure for the
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The basic freeway ramp control techniques
have not changed appreciably. Field displays
and control strategies such as pretimed
metering, traffic responsive metering, and
system metering algorithms are still valid but
with the increased computing and data
transmission techniques, those algorithms
can operate faster and virtually in real-time
leading to more efficient control and
evaluation. The techniques described below
have been drawn from previous handbooks
and updated as necessary to reflect changing
techniques.
5.3 TECHNIQUES AND
TECHNOLOGIES
The great majority of improvements and
innovations in freeway traffic management
have been in the area of computing and
communications technology capability.
Computers are faster, have more memory
and storage capability, and are more user
friendly, and virtually every person involved
in freeway management has ready access to
a personnel computer. Development of
improved communications technology has
paralleled development of the more capable
computers. Broad band fiber optic cable,
which accommodates both high speed digital
data and video, has become the standard in
most freeway management systems, rather
than twisted-pair and coaxial cable for hubto-hub transmission. Wireless technology
(such as cellular, microwave, packet radio,
and other media) has provided a means for
quick implementation until the more capital
intensive construction of fiber can be funded.
Many systems operate with a hybrid
communication system that combines
multiple media including leased telephone
lines and fiber cable.
The freeway
management techniques and strategies
documented in the 1983 Freeway
Management Handbook were much the
same as those documented in the 1985
Traffic Control Systems Handbook, although
newer technologies were described.(5,6)
ENTRANCE RAMP CONTROL
Ramp Closure
Entrance ramp closure is a seldom-used
technique except on a short term basis, and
is included here for information purposes,
and should not be considered comparable to
other ramp control techniques. The closure
of an entrance ramp during peak traffic
conditions is the simplest and most positive
form of entrance ramp control. It is also the
most restrictive. Therefore, it is usually the
least popular and it is also subject to
considerable public opposition. However, it
has been used successfully as part of a
system in a number of cities in the United
States and Japan (e.g., Houston, Los
Angeles, San Antonio, and Fort Worth, and
Osaka and Tokyo, Japan). Closure has also
been effectively used in single spot
improvements at entrance ramp applications,
such as on freeways in Beaumont and
Corpus Christi.(6) Closure may be the
appropriate measure where an entrance ramp
introduces serious weaving problems.
Although this type of entrance ramp control
can provide the same operational benefits to
freeway traffic as the other types, it lacks
flexibility.
Consequently, if applied
inappropriately, it can result in underutilizing
freeway capacity, with the consequent
overloading of alternate routes.
The 1996 Traffic Control Systems
Handbook
documented
further
developments
in
computing
and
communications hardware which had
application in freeway management.(7) Other
modules of this handbook specifically
address hardware and software that have
application in the freeway management
arena.
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Application
C
Temporary Closures. Entrance ramps
may be closed temporarily in response to
maintenance or construction activities
either on the freeway or the adjacent
frontage road or surface street. It is not
uncommon for a ramp to be closed by
police during management of a
downstream incident.
C
Variable Schedule. Because of extreme
recurring
downstream
capacity
deficiencies, ramps may be closed during
certain peak periods and open at offpeak times.
C
Permanent Closure. A ramp may be
closed on a permanent basis due to
changes in the freeway systems or
demand patterns. Concrete barriers or
other
physical
constraints
are
recommended.
Because of its limitations, entrance ramp
closure should not be considered except
under the following circumstances:
C
C
Adequate storage is not available at the
entrance ramp to prevent queues of
vehicles waiting to enter the freeway
from interfering with surface street
traffic. The closure of the entrance ramp
would eliminate the storage problem.
Traffic demand on the freeway
immediately upstream of the entrance
ramp is at capacity, and an alternate
route with adequate capacity is available.
The closure of the entrance ramp would
prevent demand from exceeding capacity
on the freeway section immediately
downstream from it, and it would divert
the traffic demand at the ramp to an
alternate route. Even if the upstream
traffic demand is less than downstream
capacity, the rate at which traffic could
be allowed to enter the freeway might be
so low that it would not be possible to
control the entrance of ramp traffic
without a large number of violations. In
this case, it would be more practical to
close the ramp in order to prevent
congestion on the freeway.
Methods
Methods of entrance ramp closure that have
been used in current systems include the
following:
With regard to the second circumstance, it
should be noted that the required demandcapacity relationship could occur because of
nonrecurrent congestion as well as because
of recurrent conditions. Therefore, closure
might be used as a response to incidents on
the freeway, as is done in Japan.(8,9)
C
Manually placed barriers such as cross
bucks, barrels, or cones.
C
Automated barriers such as those used at
railroad crossings.
C
Signing.
Experience in Detroit and Los Angeles has
indicated that signs alone cannot effect a
positive entrance ramp closure.(10,11)
Automated barriers enable an entrance to be
closed and opened automatically, which
tends to increase the flexibility of closure as
a means of control. Since manual placement
of barriers is labor intensive, this approach is
best suited for short-term or trial control
projects.
Ramps may be closed on a temporary basis,
on a scheduled basis, or permanently.
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capacity and the metering rate does not
create excessive queuing).
Ramp Metering
Metering is a method of regulating traffic
flow. When applied as a form of entrance
ramp control, metering is used to limit the
rate at which traffic can enter a freeway.
Maximum practical single lane rate is
generally at 900 vph, with practical minimum
of 240 vph. When the metering rate is not
directly influenced by mainline traffic
conditions, the control is referred to as
“pretimed metering.”
This does not,
however, necessarily imply the absence of
vehicle detectors.
For example, in the situation shown in figure
5-2, the upstream demand is 5,100 vph, the
downstream capacity is 5,400 vph, and the
ramp demand is 500 vph. Since the total
demand (5,600 vph) is greater than the
downstream capacity, ramp metering might
be a feasible solution. Therefore, if a
metering rate equal to the difference
between upstream demand and downstream
capacity (300 vph) were used, the freeway
would be able to accommodate the upstream
demand and maintain uncongested flow
while also handling 300 vph of the ramp
demand.
Metering Rates
The calculation of metering rates depends on
the purpose for which the metering is being
used. Normally, metering is used either to
eliminate congestion on the freeway or to
improve the safety of the merging operation
as follows:
However, the ultimate test of the feasibility
of ramp metering at a rate of 300 vph would
involve consideration of the following
questions:
Congestion. If the metering system is
intended to eliminate or reduce congestion,
demand must be kept at less than capacity.
Therefore, the calculation of the metering
rate at a ramp would be based on the
relationship between upstream demand,
downstream capacity, and the volume of
traffic desiring to enter the freeway at the
ramp.
Downstream capacity may be
determined by the merging capacity at the
ramp or by the capacity of the freeway
section downstream. Of course, if the sum
of upstream demand and ramp demand is
less than or equal to downstream capacity,
metering is not needed to prevent
congestion. On the other hand, if the
upstream freeway demand alone is greater
than downstream capacity, metering at the
ramp would not eliminate congestion.
Otherwise, the desired metering rate is set
equal to the difference between upstream
demand and downstream capacity (assuming
upstream demand is less than downstream
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C
Is adequate additional capacity available
in the corridor for the 200 vph that are
likely to be diverted? And, if so, is it
likely that the 200 vph would utilize that
extra corridor capacity? If not, capacity
would have to be added to the corridor
and/or made more attractive for this
number of vehicles per hour to be
diverted. Otherwise, ramp metering
would solve only the problem on the
freeway.
C
Is adequate storage available at the ramp
to accommodate the queue of vehicles
that would have to wait at the ramp
before entering the freeway? If adequate
storage could not be provided at the
ramp, alternatives to be considered
would be closure of the ramp, or
metering at other ramps upstream to
reduce upstream demand, which would
in turn permit a higher metering rate and
require less storage at the ramp.
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Figure 5-2. Example of Pretimed Entrance Ramp Metering Rate Calculation.
C
Is the specified metering rate (300 vph)
too restrictive? If so, consideration
should be given to closing or metering
other ramps upstream to reduce
upstream demand, which would permit a
higher metering rate at the ramp.
However, metering other ramps
upstream
might
lead
to
the
underutilization of the freeway.
0.5 to 1.3 seconds. Some systems use
the checkout detector (a pulse detector)
to signal the controller to terminate
green.
Signal Timing. Given that a metering rate
has been set, specific signal timing
parameters must be determined. (See figure
5-3 for general detector positioning.)
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C
Signal Cycle. Cycle is the inverse of the
metering rate or forced headway
between released vehicles. For example,
a 10-second metering rate results in a
6-second cycle or headway between
released vehicles.
Minimum Green. The green interval is
just long enough to allow one vehicle to
cross the stop line at the signal, usually
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C
Clearance Interval.
If a yellow
clearance interval is used, it is typically
0.7 to 1.0 seconds, making the total
green plus yellow 1.2 to 2.3 seconds. If
no yellow clearance is used, the 1.0
second clearance is added to the
minimum green to ensure safe clearance.
C
Red Interval. The red interval is, then,
the difference between the total cycle
length and the green plus yellow or the
green only interval.
C
Queue Detector. If the queue defector
(a presence loop) is occupied more than
some maximum length of time (say 2.0
seconds) indicating an excessive queue,
the controller may increase the metering
rate in order to reduce or clear the
queue.
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Figure 5-3. Pretimed Ramp Metering Layout.
C
before the following vehicle approaches the
merge area. The time it takes a vehicle to
merge depends on the following factors:
Merge Detector. If the merge detector
(a presence loop) is occupied more than
some maximum length of time (say 3.0
seconds), indicating the merging area is
blocked, the controller may hold the
ramp signal in red to avoid stacking on
the ramp.
The settings given above are typical but
should be fine tuned in the field to account
for unique geometrics, grades, driver
characteristics, vehicle mix, and other
factors.
Safety. If metering is to be used primarily as
a means of improving the safety of the
merging operation, then the metering rate is
simply set at a maximum consistent with
merging conditions at the particular ramp.
The primary safety problem of the merging
operation is incidence of rearend and lanechange collisions caused by platoons of
vehicles on the ramp competing for gaps in
the freeway traffic stream. Therefore,
metering is used to break up these platoons
and to enforce single-vehicle entry. For this
to happen, the metering rate selected must
ensure that each vehicle has time to merge
C
Distance the vehicle is stopped from the
freeway.
C
Geometrics of the ramp (grade, sight
distance, and length of the acceleration
lane).
C
Type of vehicle.
C
Availability of acceptable gaps in the
freeway traffic stream.
If the average time to merge is 6 seconds,
the metering rate will be 600 vph or
10 vpm.
Pretimed Metering
Pretimed metering refers to a fixed metering
rate that is not influenced by current mainline
traffic conditions. The rate will normally be
set on the basis of historical data. However,
pretimed metering does not necessarily imply
the absence of detectors.
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•
System Components
Pretimed metering is the simplest form of
entrance ramp metering.
Typical
components are shown in figure 5-3 and are
described below.
C
Ramp metering signal. Usually a
standard 3-section (red-yellow-green),
or 2-section (red-green) signal display
that controls the ramp traffic.
C
Local controller. Frequently a standard
pretimed or Type 170 controller with
capability to vary metering rates by time
of day or to accept detector inputs.
However, national cooperative efforts
among industry, operating agencies, and
FHWA have developed a design for an
open architecture protocol for local
controller.
This controller, more
properly termed a “field processor,”acts
as a communication processor with
extended capability for other freeway
management functions such as control of
variable message signs, detector
processing, and closed-circuit television
cameras.
C
C
In some applications of pretimed metering a
check-in detector is placed on the approach
to the ramp metering signal so that the signal
will remain red until a vehicle is detected at
the stop line, as shown in figure 5-3.(12)
When a vehicle is detected by the check-in
detector, the ramp metering signal will
change to green, provided the minimum red
time has elapsed. With this type of
operation, it is desirable to have a minimum
metering rate (e.g., 3 vpm) at which the
signal is set in case there is no detector
actuation, because of possible detector
failure or because of vehicles stopping too
far back from the stop line to actuate the
detector. In some cases, two detectors are
used to provide redundancy to reduce the
impact of detector failures.
In some systems, a checkout detector has
been used to ensure single-vehicle entry.
When a vehicle is permitted to pass the ramp
metering signal, it is detected by the
checkout detector, which is installed just
beyond the stop line (usually about half a car
length past it). The green interval is then
terminated as soon as the vehicle is sensed
by the checkout detector. In this way, the
length of the green interval is made sufficient
for the passage of only one vehicle.
Advance ramp control warning sign
with flashing beacon. A sign which
indicates to traffic approaching the ramp
that it is being metered. In California, a
blank-out type "METER ON" sign is
used at many installations in lieu of the
flashing beacon.
In some pretimed metering systems, a queue
detector is used to detect backing of ramp
traffic into frontage roads or surface streets.
The queue detector is placed at a strategic
point on the ramp, or on the frontage road,
in advance of the ramp metering signal.
When a queue is sensed by a vehicle
occupying the loop for a selectable period of
time, indicating that the queue of vehicles
waiting at the ramp metering signal is
sufficient to interfere with traffic on the
frontage road or surface streets, a higher
metering rate may used to shorten the queue
length. This can be self-defeating, however,
Vehicle Detectors. There are four types
of detectors that are generally used with
this type of ramp metering strategy:
•
Check in (demand) detectors.
•
Checkout (passage) detectors.
•
Queue detectors.
Merge detectors.
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since shorter queues often attract higher
demands.
to allow one vehicle to proceed past the
signal. The red interval varies with the
number of vehicles being metered. For
instance, if a metering rate of 600 vph or 10
vpm were to be used, and the green-plusyellow interval were 2 seconds, a red interval
of 4 seconds would be used. If the metering
rate were 300 vph, or 5 vpm, and the greenplus-yellow interval were 2 seconds, a red
interval of 10 seconds would be used.
A merge detector is a device that senses the
presence of vehicles in the primary merging
area of the ramp and freeway mainlanes.
When the merge detector senses that a
vehicle has stopped, blocking the merge
area, the signal may be held in red for some
preset maximum time in order not to clog
the area and to reduce the possibility of a
rear end collision. On a well designed
entrance ramp with adequate acceleration
and merging distance, a merge detector is
not necessary or practical.
Platoon metering. When metering rates
greater than 900 vph are required, platoon
metering, which permits the release of 2 or
more vehicles per cycle, may be used to
achieve such high metering rates. For
pretimed platoon metering, the cycle length
is determined on the basis of the desired
metering rate and the average number of
vehicles to be released per cycle. For
example, in the case of a metering rate of
1,080 vph, or 18 vpm, and a release of 2
vehicles per cycle, 9 cycles per minute would
be required. Therefore, the cycle length
would be 6.67 seconds. Similarly, if a
release of 3 vpc were used instead, the cycle
length would be 10 seconds. However, the
timing of the cycle intervals (i.e., green,
yellow, red) would depend on the form of
platoon metering used, tandem or 2-abreast.
Placement of these auxiliary detectors is
discussed in more detail in the subsequent
section on traffic responsive metering.
Figure 5-4 shows ramp metering signals and
advance warning signs that have been used.
Also, for a discussion of standards for
various system components, refer to the
recommended practice for freeway entrance
ramp displays prepared by the Institute of
Transportation Engineers (ITE).(13)
System Operation
In the operation of a pretimed metering
system, the ramp signal operates with a
constant cycle in accordance with a metering
rate prescribed for the particular control
period. However, timing the red, yellow,
and green intervals of the cycle (many
systems use ramp signals that have only red
and green intervals) depends on whether the
type of metering used is single-entry
metering or platoon metering.
Tandem Metering. In the case of tandem
metering, the vehicles are released one after
another. Therefore, the green-plus yellow
time is made long enough to permit the
clearance of the desired number of vehicles
per cycle. A yellow interval should be used
to minimize the rearend collision potential.
Thus, for the 7-second cycle with 2-vehicle
platoons, a 4-second green-plus-yellow and
a 3-second red might be used. And for a 12second cycle with 3-vehicle platoons, a 9second green-plus-yellow and a 3-second red
might be used. Experience indicates that
2-vehicle platoons can be handled
satisfactorily and that 3-vehicle platoons are
a practical maximum. In either case, a
Single-entry metering. In the case of
single-entry metering, the ramp metering
signal is timed to permit only one vehicle to
enter the freeway per green interval.
Therefore, the green-plus-yellow (or just
green if yellow is not used) interval is just
long enough (usually about 1.5 to 2 seconds)
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Figure 5-4. Typical Field Displays for Ramp Meter Installations.
maximum metering rate of 1,100 vph can be
expected.(14)
remainder of the cycle is red. With alternate
release metering, maximum metering rates of
about 1,700 vph may be achieved.
Two-abreast Metering. With two-abreast
metering, two vehicles are released side by
side per cycle. This form of metering
requires two parallel lanes on the entrance
ramp plus a sufficient distance beyond the
ramp metering signal for the two vehicles to
achieve a tandem configuration before
merging with freeway traffic. The more
common practice in two-lane situations is to
alternate the release—one from the left lane
followed by one from the right. The timing
of the cycle intervals for multiple-lane
metering is similar to that for single-entry
metering in that the green-plus-yellow
interval is just long enough (usually about 3
seconds) to allow one vehicle in each lane to
proceed past the ramp metering signal. The
Compared to single-entry metering, platoon
metering is a more complex operation and
may cause some drive confusion which may
lead to disruptions of ramp flow. Therefore,
single-entry metering should always be given
first consideration, and platoon metering
should not be used unless it is necessary to
achieve higher metering rates. However,
platoon metering has been successfully used
in several locations and drivers can adapt
with proper design and pre-operation
publicity.
It has been shown that entrance ramp control
can be extremely cost effective.(1)
Experience has indicated that the biggest net
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gain in benefits is realized in going from no
control to pretimed metering. Pretimed
metering offers both advantages and
disadvantages.
The most important
advantages are that it gives the driver a
dependable situation to which he can readily
adjust, and that it tends to be associated with
lower costs. The major disadvantage is that
the system can neither respond automatically
to significant changes in demand, nor adjust
to unusual traffic conditions resulting from
incidents. Because of this inability to
automatically respond to changes in traffic
conditions and the relative difficulty of
dissipating resultant congestion, pretimed
metering rates have usually been set so that
operation will be at volumes slightly below
capacity at the desired LOS.
C
At zero density, or when no vehicles are
on the roadway, the flow rate is zero.
and traffic is permitted to travel at its
free speed, uf.
C
As density increases to a value, km, the
flow rate increases to a maximum value,
qm, which is the capacity of the roadway,
and speed decreases to a value, um.
C
As density increases from a value, km, to
a maximum value, kj (jam density), the
flow rate decreases to zero because the
roadway is blocked with too many
vehicles for traffic to move.
The values of qm, uf, um, km, and kj—and the
shapes of the curves—depend on several
factors including geometrics of the roadway,
composition of traffic, and weather
conditions. Therefore, these values may be
different for different sections of the
roadway, and each section may have more
than one set of these values. Although these
are theoretical relationships based on the
assumption of uniform traffic flow, the
trends expressed by these relationships do
exist.(16)
Traffic-Responsive Metering
In contrast to pretimed metering control,
traffic-responsive metering is directly
influenced by the mainline and ramp traffic
conditions during the metering period.
Metering rates are selected on the basis of
real-time measurements of traffic variables
indicating the current relation between
upstream demand and downstream capacity.
Basic Strategy
Fundamental Traffic Flow Relationships
As explained earlier, congestion occurs
whenever demand exceeds capacity.
Therefore, as indicated in figure 5-5, the
values of qm, um, and km define boundaries
between congested flow and uncongested
flow. The purpose of metering is to prevent
or reduce congestion, or in other words, to
keep the values of the fundamental traffic
flow variables at levels that define points on
the uncongested-flow portions of the traffic
flow curves. Thus, the basic strategy of
traffic responsive metering is as follows:
In order to determine or predict demandcapacity conditions on the basis of real-time
measurements of traffic variables, a
description or model of traffic is necessary.
Most frequently used as indicators of
operating conditions for traffic-responsive
metering are functional relationships
between flow rate, q; space-mean speed, u;
and density, k.(15)
A generalized relationship between each of
the variables is depicted in figure 5-5 and can
be summarized as follows:
C
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Obtain real-time measurements of traffic
variables on the freeway.
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Figure 5-5. Fundamental Flow Rate-Speed-Density Relationships.
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On the basis of these measurements,
determine where on the fundamental
traffic flow curves the freeway section is
operating with respect to capacity.
capacity used should account for the effects
on capacity of weather conditions, traffic
composition, and incidents.
The difference between the upstream volume
and the downstream capacity is then
determined and used as the allowable
entrance ramp volume. This ramp volume is
expressed as a metering rate to be used
during the next control interval (usually 1
min). If the upstream volume is greater than
the downstream capacity, a minimum
metering rate is used (e.g., 3-4 vpm).
Theoretically, if the upstream volume were
greater than the downstream capacity, a zero
metering rate, or ramp closure, should be
used in order to prevent congestion. It has
generally been found that metering rates
lower than 3 vpm are not effective, because
vehicles waiting at the ramp will judge the
ramp metering signal to be malfunctioning
and will proceed through on red.
Determine the maximum ramp metering
rate at which vehicles can be permitted
to enter the freeway.
A refinement that is often made to this
strategy is to select the traffic flow curves on
the basis of traffic composition and weather
conditions.
Lane occupancy (a surrogate measure for
density) and flow rate (volume) are the two
traffic variables generally used to describe
freeway traffic conditions for traffic
responsive metering. These are the control
parameters usually used, because they can be
measured in real time using vehicle
detectors.
Several variations on the basic strategy of
traffic-responsive metering utilize different
combinations of traffic variables. Although
most are reported as having positive effects
on freeway operations, none has been
generally accepted as being superior to the
others. In fact, new strategies are still being
formulated to find better modes of control.
However, the principal traffic-responsive
strategies remain demand-capacity control
and occupancy control.
Downstream capacity may also be measured
directly from freeway detector(s) to reflect
for variations in traffic composition,
weather, or other limiting factors which
would not be accounted for in a fixed value
of capacity.
Since a low upstream volume could occur in
congested as well as uncongested flow,
volume alone does not indicate degree of
congestion.
Therefore, an occupancy
measurement also is usually made to
determine whether uncongested or
congested flow prevails. If the occupancy
measurement is above a preset value (e.g.,
18 percent, as used in Los Angeles).(17)
which is determined from historical data,
congested flow will be assumed to exist and
a minimum metering rate used.
Demand-Capacity Control
Demand-capacity control features the
selection of metering rates on the basis of a
real-time comparison of upstream volume
and downstream capacity. The upstream
volume is measured in real time and
compared with either a preset value of
downstream capacity determined from
historical data or a real-time value computed
from downstream volume measurements.
To be most effective, the downstream
Occupancy Control
Occupancy control utilizes real-time
occupancy measurements generally taken
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upstream of the entrance ramp. One of a
number of predetermined metering rates is
selected for the next control interval (usually
1 min) on the basis of occupancy
measurements taken during the current
control interval. For a given entrance ramp,
the metering rate to be used for a particular
value of occupancy would be based on a plot
of historical volume-occupancy data
collected at each measurement location. An
example of a typical plot from Chicago is
shown in figure 5-6.(18) From such a plot, an
approximate relationship between volume
and occupancy at capacity is determined.
For each level of occupancy measured, a
metering rate can be determined that
corresponds to the difference between the
predetermined estimate of capacity and the
real-time estimate of volume.
If the
measured occupancy is greater than, or equal
to, the preset capacity occupancy, a
minimum metering rate will be selected
instead of a zero rate or ramp closure. This
choice would be based on effective and
practical
entrance
ramp
control
considerations, as explained earlier for
demand-capacity control. Table 5-1 shows
a recommended range of metering rates
based on measured occupancy.(19)
System Components
A traffic-responsive metering system
contains the same components as described
for pretimed metering. These include ramp
metering signal(s), local controller, advance
warning sign with flashing beacon, and
detectors. The local controller unit for
traffic-responsive
metering
requires
additional logic over and above that required
for pretimed metering in order to monitor
traffic variable measurements, select or
calculate metering rates, and respond to
override-type conditions such as excessive
queues. Queue, check in, checkout, and
merge detectors are normally also included
in traffic-responsive metering systems.
Some traffic-responsive metering systems
have also included detectors used to
determine traffic composition and weather
conditions.(9, 17) Input from these detectors
enables the system to account for the effects
of these factors on traffic flow.
Table 5-1. Local Actuated Metering Rates as Function of Occupancy. (19)
Occupancy (%)
Metering Rate(Vehicles/ Minute)
< 10
12
11-16
10
17-22
8
23-28
6
29-34
4
> 34
3
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Figure 5-6. Typical Volume-Capacity Plot Related to Ramp Metering Rate. (18)
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The typical location of these components on
a ramp is shown in figure 5-7. For a
discussion of standards for various
components, the reader is referred to the
publication on recommended practice for
freeway entrance ramp control displays
prepared by the Institute of Transportation
Engineers (ITE).(13)
System Operation
Metering Rate Selection. Single-entry
metering is normally used to time the
red-yellow-green (or red-green) intervals
for a given metering rate. However, if
high metering rates (e.g., higher than 13
vpm), are required, platoon metering
might be used. Single-entry metering
and platoon metering should not both be
used at any one ramp.
In a traffic-responsive metering system,
the selection is based on real-time
measurements of traffic variables which
describe traffic flow conditions on the
freeway. The control interval, which is
the time period during which a selected
metering rate remains in effect, is much
shorter for a traffic-responsive metering
The operation of a traffic-responsive
metering system is similar to that of a
pretimed metering system, except in regard
to the following:
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Figure 5-7. Traffic Responsive Ramp Metering Layout.
system (e.g., 1 min) than for a pretimed
metering system (e.g., 30 min., 1 hr., or the
entire peak period).
ramp metering signal is returned to or
left in green.
C
C
Override Features. Override features
of a traffic responsive system adjust
metering rates in accordance with certain
operational considerations as follows:
C
Continued Actuation of the Queue
Detector. Indicates that the queue of
vehicles waiting at the ramp metering
signal is approaching the frontage road
or surface street and is likely to interfere
with traffic on either or both. Therefore,
a higher metering rate may be used to
shorten the queue length.
C
Actuation of the Merge Detector.
Indicates that a vehicle is still in the
merge area. Therefore, in the case of
single-entry metering, subsequent green
intervals are preempted until the vehicle
merges.
C
No Actuation of the Checkout
(Passages Detector After a Green
Interval). Indicates that a vehicle has
missed the green signal. Therefore, the
Continued Actuation of the Queue
Detector With No Actuation of the
Check in (Demand) Detector. Such a
condition indicates that a vehicle on the
ramp has stopped short of the check in
detector. Therefore, the ramp metering
signal is turned to green to allow this
vehicle to proceed.
Gap-Acceptance Merge Control
Gap-acceptance merge control has been
implemented and tested, but is little used, if
at all, today. The concept of matching a
merging vehicle to a specific freeway gap is
attractive, but many variables can cause it to
fail. Certain elements, such as slow vehicle
detection, may have application in other
types of ramp control operation. Gapacceptance merge control might have
application
where
geometries
are
substandard and the primary concern is
safety.
The merge-control concept of entrance ramp
metering is intended to enable a maximum
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number of entrance ramp vehicles to merge
safely without causing significant disruptions
in freeway traffic. The concept involves
maximum utilization of gaps in the traffic
stream of the freeway lane into which ramp
vehicles are to merge. It may or may not
involve the calculation of ramp metering
rates in accordance with the demandcapacity constraint. The problem is mainly
one of inserting entrance ramp vehicles into
freeway gaps. However, a provisional
metering rate based on system calculations
may be established. If a gap is found in a
“window”, say 3 seconds either side of the
calculated release point, it is considered to
have satisfied the metering rate, and a
vehicle is released.(20)
Gap acceptance
metering has not been widely used, but may
be warranted where geometrics are
substandard or the safety of the merging
operation can be improved.
Merge-control systems are designed to
improve the merging operation at the
entrance ramp by providing the driver with
the information needed to coordinate in time
and space entry onto the freeway. These
systems operate according to the following
basic guideline procedures:
Basic Concepts
The concepts of gap acceptance at freeway
entrance ramps are important in describing
the interaction of the freeway and ramp
traffic. It is assumed that the ramp driver
measures each gap in the adjacent freeway
lane and compares it with an acceptable gap
which he/she judges as large enough for a
safe merge.
Entrance ramp and freeway geometrics.
C
Vehicle performance characteristics.
C
Driver behavior.
C
Traffic conditions.
C
Weather conditions.
Detection of an acceptable gap on the
freeway into which a ramp vehicle could
merge.
C
Projection of the arrival of the
acceptable gap at the merging point of
the entrance ramp.
C
Release of the ramp vehicle in sufficient
time for it to accelerate and merge into
the moving gap.
C
If a gap is not detected within some
maximum time, say 60 seconds, the
vehicle is released.
System Components
Gap-acceptance merge-control systems use
many of the same components as those
described for pretimed metering, which
include ramp metering signals, local
controller, advance warning sign with
flashing beacon, and detectors. A mainlane
gap/speed detector is located in the shoulder
lane of the freeway upstream of the ramp
merge to provide data from which the
controller unit can determine presence and
speed of available gaps in which to insert
merging ramp traffic. Queue, check in,
checkout, and merge detectors are normally
included in gap-acceptance merge-control
systems.
The minimum acceptable gap is dependent
on several factors, such as the following:(20)
C
C
Another override-feature component that
might be added to the system is a slowvehicle detector, which senses the presence
of a slow-moving vehicle on the entrance
ramp between the ramp metering signal and
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the merge detector. A schematic layout for
gap-acceptance operation combined with
traffic responsive operation as implemented
in the Dallas Corridor Study is presented
in figure 5-8.(22) Also, for a discussion of
standards for various system components,
the reader is referred to the publication on
recommended practice for freeway entrance
ramp control displays prepared by the
Institute of Transportation Engineers
(ITE).(13)
C
The controller begins to measure gaps
and vehicle speeds which are sensed by
the gap/speed detector that is located
upstream from the ramp in the lane of
the freeway into which ramp vehicles are
to merge.
C
The controller compares each measured
gap to a preset minimum gap size to
determine whether or not it is an
acceptable gap.
System Operation
C
A gap-acceptance merge-control system
does not normally operate in accordance
with a constant metering rate for a specified
control interval as do pretimed and trafficresponsive metering systems. Instead, it
operates in response to the availability of
acceptable gaps in the lane of the freeway
into which ramp vehicles are to merge.
If the system includes demand-capacity
features as described above, the
controller determines if the gap falls
within a “window” and adjusts the
release time accordingly.
C
If a gap is not acceptable, the controller
considers the next gap.
If it is
acceptable, the controller computes the
time at which the vehicle at the ramp
metering signal should be released in
order to arrive at the merge point at the
same time as does the acceptable gap.
This calculation involves the following
factors:
Usually, the system is designed to operate in
a single-entry metering mode, with the ramp
metering signal resting on red when no
vehicles are waiting on the ramp.
Experience on the Gulf Freeway in Houston
has indicated that it is usually not desirable
to operate the ramp metering signal in either
of the following two ways:(24)
C
If it gives a green indication at the
proper time, whether or not there is a
vehicle waiting.
C
If it normally rests on green when there
are no vehicles waiting.
Procedures can be summarized as follows
for the nominal operation of a gapacceptance merge-control system, with
single-entry metering and the ramp metering
signal resting on red:
C
A vehicle stops at the ramp metering
signal and actuates the check in detector.
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-
Speed of the traffic flow measured in
the lane of the freeway into which
ramp vehicles are to merge.
-
Distance of the gap/speed detector
location from the merge point.
-
Predetermined ramp travel time of a
vehicle stopped at the ramp metering
signal to the merge point.
C
At the proper instant, the controller
causes the ramp metering signal to
change to green.
C
The ramp metering signal remains on
green for a fixed interval long enough to
release a single vehicle. Then, it changes
to yellow for a short fixed interval before
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Figure 5-8. Gap Acceptance / Traffic Responsive Ramp Metering Layout.
it changes to red. (Where permitted by State
law, the yellow interval may be omitted.)
The green plus yellow (or green only)
interval is usually about 3 seconds long. It is
necessary that the ramp metering signal
remain on red long enough to give the next
vehicle in line time to pull up to the signal.
Thus, the minimum time for a full greenyellow-red (or green-red) cycle should be 4
to 5 seconds.
successive vehicles constitute large time
headways because of the low speeds.
For example, if traffic on the freeway
should come to a complete stop, the
measured time headways will be
infinitely large.
Thus, unless an
appropriate override were provided, the
controller would release a number of
entrance ramp vehicles to enter the
freeway during the congested flow, a
response which would be contrary to the
objective of improving freeway
operations. Therefore, if the speed of
the freeway traffic drops below a preset
value (e.g., 25 mi/h), ramp vehicles are
metered at a minimum fixed rate {usually
3 to 4 vpm).
The operation of the override features of a
gap-acceptance merge-control system is
essentially the same as for a trafficresponsive metering system. However, a
gap-acceptance merge-control system may
have the following additional override
features:
C
C
Low-speed, Fixed-rate Metering.
When congested flow develops on the
freeway, small space headways between
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Slow-vehicle, Red-interval Extension.
At entrance ramps where there are
relatively high percentages of trucks and
buses, it might be desirable to make
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special allowances for their performance
characteristics. Accordingly, a slowvehicle detector might be provided to
measure the travel time of vehicles from
the ramp metering signal to their
location. If the travel time is greater
than a preset value, the ramp metering
signal is held on red until the vehicle has
cleared the merge detector or until the
merge detector is actuated.
System Ramp Control
System ramp control refers to the application
of ramp control to a series of entrance ramps
where a single ramp meter cannot address
the excess freeway demand. The primary
objective of system ramp control is to
prevent or reduce the occurrence of
congestion on the freeway. Therefore, the
control of each ramp in the control system is
based on the demand-capacity considerations
for the whole system rather than on the
demand-capacity constraint at each
individual ramp. This concept does not
necessarily imply the use of large computer
control systems, since small subsystems may
be coordinated by the use of mutual
coordination of adjacent ramp meter
controllers.
Benefits to be realized from a gapacceptance merge-control system are similar
to those realized from traffic-responsive
metering system. A study conducted by the
Texas Transportation Institute, which
compares a gap-acceptance merge-control
system with a demand-capacity-control,
traffic-responsive metering system, has
reported the following results:(23)
C
C
C
If congestion is to be prevented or reduced
on the freeway system, the concept of
system ramp control must be used in the
design of a system of controls for a section
of freeway with more than one entrance
ramp. It may be applied in the following
types of systems:
Gap-acceptance merge-control resulted
in a higher percentage of ramp-meteringsignal violations by ramp vehicles, which
was probably due to its irregular pattern
of operation and longer queue delays
(metering rates ranged from 1 veh/4
seconds to 1 veh/25 seconds).
Gap-acceptance merge-control resulted
in lower travel times from the ramp
metering signal to the merge area, which
indicates a smoother merging operation.
Demand-capacity control resulted in
higher metering rates and higher peakhour entrance ramp volume.
C
System pretimed metering (including
ramp closure).
C
Traffic-responsive metering.
C
Gap-acceptance merge control.
A discussion of system ramp control applied
to each of these systems follows.
In general, for entrance ramps that have
well-designed geometrics, a gap-acceptance
merge control is less cost-effective than
either pretimed or traffic-responsive
metering systems. However, gap-acceptance
control might be warranted at locations
where the geometrics are substandard and
the primary concern is to improve the safety
of the merging operation.
System Pretimed Metering
System pretimed metering refers to the
application of pretimed metering to a series
of entrance ramps. The metering rate for
each of these ramps is determined in
accordance
with
demand-capacity
constraints at the other ramps as well as its
own local demand-capacity constraint.
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Determining these metering rates, which are
computed from historical data pertaining to
each control interval, requires the following
information:
C
Mainline and entrance ramp demands.
C
Freeway
capacities
immediately
downstream of each entrance ramp.
4. Compare the upstream mainline demand
to the capacity of the downstream
section and proceed as follows:
a. If the upstream mainline demand is
less than the capacity, then the
allowable entrance ramp volume (or
metering rate) is set equal to the
difference between the capacity and
the upstream mainline demand.
C
Description of the traffic pattern within
the freeway section to be controlled.
This information provides the basis for
establishing the demand-capacity constraints
of the entrance ramps and their
interdependencies.
b. If the upstream mainline demand is
greater than or equal to the capacity,
then the allowable entrance ramp
volume is zero, and the ramp must
be closed. If the upstream mainline
demand is greater than the capacity,
the volumes permitted to enter at
ramps upstream must be reduced
accordingly. The total reduction in
the allowable entrance ramp volumes
upstream is equal to the difference
between the upstream mainline
demand and the capacity, adjusted to
account for that portion of the traffic
entering upstream that exits before it
reaches the downstream entrance
ramp being closed.
Fundamental
metering
rate
calculations—given the required data, the
fundamental procedure for computing
metering rates involves five steps:
1. Start with the entrance ramp that is
farthest upstream.
2. Determine the total demand (upstream
mainline demand plus ramp demand) for
the freeway section immediately
downstream of the ramp.
5. Select the next entrance ramp
downstream and go back to step 2.
3. Compare the total demand to the
capacity of the downstream section, and
proceed as follows:
This procedure is illustrated by the following
examples.
a. If the total demand is less than the
capacity, metering is not required at
this ramp by this demand-capacity
constraint. Therefore, skip step 4
and go immediately to step 5.
Example 1 (5,6)
In the example case shown in figure 5-9,
pretimed metering rates are calculated for an
integrated, pretimed control system
comprised of four entrance ramps. In
reviewing this example, the following points
should be noted:
b. If the total demand is greater than
the capacity, metering is required at
this ramp by the demand-capacity
constraint. Therefore, proceed to
step 5.
-
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Since only entrance ramp control is
being considered and not mainline
control, the allowable mainline volume at
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Figure 5-9. Integrated Entrance Ramp Control: Example No. 1 Calculation of
Pretimed Metering Rates. (5,6)
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Section 1, Xj is set equal to the mainline
demand Dj.
-
The demand, S2, at Section 2 is 5,570 vph,
which is 770 vph greater than the capacity,
B2, at Section 2 (4,800 vph). If Ramp 2 is
closed, the demand at Section 2 is reduced
to 4,970 vph, a volume which also exceeds
the capacity, B2. Therefore, it is necessary
to reduce the allowable volume, X2, entering
at Ramp 1 (input 2). The allowable volume,
X2 , must be reduced enough to reduce the
demand, S2, by 170 vph. The amount of the
reduction is equal to the 170 vph divided by
the decimal fraction, A22, of the vehicles
entering at Ramp 1 and passing through
Section 2 (170 vph/0.75 - 227 vph).
Therefore, the allowable volume, X2, at
Ramp 1 would be 573 vph instead of
800 vph.
With the notation given in figure 5-9, the
demand, Sj, at a section, j, is computed
by the following equation:
S j ' ( j Aij X i) % Aj%1Dj%1
j
i'1
where:
Xi =
Allowable volume at input i
Dj=
Demand at input i
Aij =
Decimal fraction of vehicles
entering at input i which pass
through Section j
As it happens, the metering rate computed
for each entrance ramp in this particular
example is determined solely by the demandcapacity constraint at the section
immediately downstream and is not
influenced by the demand-capacity
constraints at other ramps.
In this procedure, excess demand, Sj - Bj, at
any section, j, is removed by reducing the
allowable volume on the entrance ramp
immediately upstream. If' instead, the
allowable volumes on entrance ramps farther
upstream were reduced, a large number of
vehicles would have to be removed from
these ramps in order to reduce the demand,
Sj , sufficiently at any section, j. This is
necessary because some of the vehicles that
enter at these ramps will exit the freeway
before they reach Section j.
Example 2(5,6)
Example 3
The data given in the example shown in
figure 5-10 are the same as those given in
the previous example. except that the
mainline demand, D1, is 4,600 vph instead of
4,000 vph. In this case, the metering rates at
Ramps 2, 3, and 4 are determined solely by
their respective downstream demandcapacity constraints, as was the case in the
previous example. However, the metering
rate at Ramp 1, rather than being determined
by the demand-capacity constraint at Section
1, is established in accordance with the
demand-capacity constraint at Ramp 2, as is
described below.
Again, in the situation presented in table 5-2,
allowable ramp volumes would be calculated
as follows. If the excess demand, 1200 vph,
at Section 2 were to be removed by reducing
the allowable volume, X2, at Ramp 1, the
volume at Ramp 1 would have to be reduced
by 267 vph. The allowable entrance ramp
volumes are summarized accordingly in table
5-2.
Sj=
Demand at Section j
The total input of 2,172 vph, however, is
less than that of 2,248 vph, the volume
obtained if Ramp 2 is metered as in Example
1. Thus, the fundamental approach
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Figure 5-10. Integrated Entrance Ramp Control: Example No. 2 Calculation of
Pretimed Metering Rates. (5,6)
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Table 5-2. Allowable Entrance Ramp Volumes for
Example 3.
Ramp No.
Volume (vph)
1
533
2
600
3
687
4
352
Total Input
2172
described will result in the optimal utilization
of the freeway. It maximizes the sum of the
allowable entrance ramp volumes, a
procedure which corresponds to maximizing
system output for steady-state, uncongested
flow conditions.(26) It also maximizes the
total travel in the system.(27)
-
At Section 1, allowable mainline
volume < mainline demand:
X1 = D1
-
Linear Programming Formulation—The
fundamental procedure described in
Examples 1 and 2 can be formulated as a
linear programming model.(26) This model
may be used to compute optimal allowable
entrance ramp volumes. In terms of the
notation defined in figures 5-8 and 5-9, the
linear programming model would be as
follows:
Allowable entrance ramp volume >
entrance ramp demand:
X1 < D1; i=2,.....,n
-
Allowable entrance ramp volume
equals minimum allowable ramp
volume:
X1 > min xi >0; i = 2,.....n
C
Maximize ΣXj, where n is the number of
inputs
The use of the linear programming model
yields allowable entrance ramp volumes
identical to those obtained by using the
fundamental procedure described above.
C
Subject to the following constraints:
Practical Considerations
-
The allowable entrance ramp volumes (or
metering rates) calculated for an integrated
ramp control system should be evaluated
with respect to the following practical
considerations:(6)
Demand capacity:
j Aij Xi # Bj ; j'1,.....n&1
n
i
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Manual TABLE OF CONTENTS
C
C
Metering rates of less than 180 to 240
vph (3 to 4 vpm) are not feasible because
drivers required to wait longer than 15
to 20 seconds at a ramp metering signal
often believe that the signal is not
working correctly. They will, therefore,
proceed on a red indication by the signal.
Thus, if a metering rate of less than 180
to 240 vph is calculated, consideration
should be given either to closing the
ramp or to metering it at a higher rate.
necessary to increase the metering rates
computed for some of the downstream
entrance ramps, and thus to reduce
accordingly the metering rates for some
of the upstream entrance ramps.
If any of the computed metering rates were
to be altered because of one or more of the
practical considerations mentioned above,
the metering rates at the other entrance
ramps would have to be adjusted accordingly
to ensure both an optimal utilization of the
freeway and an uncongested flow.
Practical maximum metering rates are
about 900 vph for single-entry metering
and approximately 1,100 vph for platoon
metering. Therefore, for a metering rate
greater than the maximum for the
metering type to be used, the setting
should be less than or equal to the
practical maximum rate, and the
metering rates at the other entrance
ramps should be adjusted accordingly.
C
Metering rates at each entrance ramp
should be evaluated with regard to
available storage at the ramp and
potential resulting congestion on the
adjoining surface street system. If the
storage is not sufficient, it may be
necessary either to close the ramp or to
increase the metering rate.
C
Metering rates equal to zero indicate that
an entrance ramp closure is necessary.
However, the closure of a particular
entrance ramp may not be acceptable.
Therefore, it may be necessary to
increase a zero metering rate to some
minimum acceptable rate.
C
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Example 4 (5,6)
If it were necessary to maintain a metering
rate of at least 240 vph at Ramp 2 in the
example presented in figure 5-10, it would
be necessary to follow the adjustment
procedure for the metering rates at the other
entrance ramps (as shown in figure 5-11).
The allowable volume. X2 (573 vph), at
Ramp 1 would have to be reduced by
320 vph in order to allow 240 vph to enter at
Ramp 2 and still satisfy the demand-capacity
constraint at Section 2. This reduction also
decreases the mainline demand at Sections 3
and 4. Thus, the allowable volumes at
Ramps 3 and 4 are increased to maximize
the utilization of the freeway at these
sections.
It is usually difficult to obtain reliable
estimates of the Aij values, because these
vary with time and generally exhibit a high
variance. Also, the O/D type studies used to
collect these data are expensive and do not
provide real-time data.
The procedure described for computing
metering rates gives preference to traffic
entering the system near the upstream
end. Consequently, metering rates at
entrance ramps downstream may be too
restrictive to be acceptable to the
motoring public. Therefore, it may be
As indicated in the 1996 Traffic Control
System Handbook, it may be unfeasible to
reduce ramp volumes sufficiently to effect
changes on freeway main lanes because of
circumstances such as the following:(7)
C
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Minimum metering rate constraints.
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Figure 5-11. Integrated Entrance Ramp Control: Example No. 4 Calculation of
Pretimed Metering Rates. (5,6)
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C
Lack of vehicle queuing storage.
C
Too large a capacity deficiency.
Module 5. TABLE OF CONTENTS
capacity conditions expected from which the
metering rates are then selected in real time.
The linear programming model is often used
to calculate predetermined sets of integrated,
traffic-responsive reentering rates. Also, the
metering rates are usually subject to the
merge-detector,
queue-detector,
and
maximum-red-time overrides used in trafficresponsive metering
The reader is referred to the handbook for a
detailed example demonstrating the
interaction
among
ramp
metering
requirements, diversion impacts, and ramp
storage requirements. A detailed procedure
can be found in reference 19.
System vs. Independent Ramp Control
Systemwide ramp metering strategies
provide the opportunity to distribute vehicle
demands over a larger number of ramps.
Comparisons of system and independent
entrance ramp control indicate that increased
benefits are realized with system ramp
control.(26,27) Improvements occur in terms
of the following:
System Traffic-Responsive Metering
System traffic-responsive metering is the
application of traffic responsive metering to
a series of entrance ramps where the
metering rates at each ramp are selected in
accordance with both system and local
demand-capacity constraints.
C
Lower travel time.
C
Higher total travel.
C
Fewer crashes.
In traffic-responsive metering, the greater
system flexibility provided by system ramp
control enables an optimal system response
to individual variations in traffic demands
and capacities resulting from incidents on the
freeways.
System Operation
During each control interval, real-time
measurements are taken of traffic variables
(usually volume, occupancy, and/or speed).
The data are used to define the demandcapacity conditions at each ramp. Then, on
the basis of these measurements, both an
independent and an integrated metering rate
are calculated for each entrance ramp. Of
these two metering rates, the one that is the
more restrictive is selected to be used during
the next successive control interval.
Controller Interconnection
A significant feature of system ramp control
is the interconnection among local ramp
controllers, which permits conditions at one
location to affect the metering rate imposed
at one or more other locations. Real-time
metering plans are computed and updated by
a central master computer which issues
metering rates to the respective local ramp
controllers on the basis of freeway traffic
information obtained from vehicle detectors
throughout the system.
Metering Rates
The methods used to calculate independent
and integrated traffic-responsive metering
rates are basically the same as those used to
compute independent and integrated
pretimed metering rates.
Instead of
calculating metering rates in real time, a set
is precomputed for the range of demand-
Although the decision-making capabilities
are centralized within the central computer
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system, the processing of control intelligence
may be distributed among the individual
entrance ramps. For economic (and possibly
reliability) reasons, there is a trend toward
decentralized decision-making, distributed
computation, and hierarchical control. (28)
Incremental Benefits of Various Levels of
Control
As discussed earlier, the benefits offered by
pretimed metering (including ramp closure)
versus no access control include increased
mainline speeds (reduced travel time), higher
service volumes, less delay, safer merging
operations, and reduced user costs. Beyond
pretimed metering, the incremental benefits
gained from traffic-responsive metering
(local or systemwide) depend on the factors
discussed below.(21)
RAMBO (Ramp Adaptive Metering
Bottleneck Optimization) is a suite of
programs developed for the Texas
Department of Transportation by the Texas
Transportation Institute.(29) RAMBO I is a
software tool designed to assist in
developing ramp metering plans using the
TxDOT ramp meter specification, while
operating either in the isolated mode or in
local control.
The program provides
Transition Point Patterns for each metering
level and evaluates traffic operations.
RAMBO II likewise develops and evaluates
ramp metering plans based on forecasted
traffic conditions along an extended section
of freeway containing up to 12 metered
entrance ramps and 12 exit ramps operating
either in the system mode, or in a
hierarchically distributed system having realtime local control with systems-based
metering objectives. The program was
implemented in Houston, TX, in 1996.
Variations in the Ratio of Mainline to
Entrance Ramp Demand
As mainline demand approaches capacity,
the permissible metering rates become more
and more constrained. On the other hand, as
the mainline demand decreases. more traffic
can be allowed onto the freeway from
entrance ramps, and ramp metering control
can exert greater impact on the quality of
freeway flow, thus producing greater
benefits.
Variations in Overall Traffic Demand
Pattern
The total software package can perform
capacity analysis of the freeway system,
assess projected metering operation, and
assist in developing optimal ramp metering
plans for either local ramp metering
operations (using RAMBO I) or system
ramp metering operations (using RAMBO
II). RAMBO II can translate system-based
results into local metering control
parameters that can be downloaded into the
local ramp meters if some minor
modifications are made to the current ramp
meter specifications. The programs include
extensive interactive graphic screens.
Traffic demand on the freeway and entrance
ramps exhibits two types of variations: (1)
shift in demand level, and (2) short-term
fluctuations. The larger the magnitude of
these types of demand variation, the higher
the potential for benefits from trafficresponsive metering.
Mainline Capacity Reductions
Reductions in mainline freeway capacity
result from accidents, traffic incidents, and
adverse weather conditions.
As the
frequency and impact of these capacityreducing factors increase, more need
develops for traffic-responsive metering to
cope with the variations in available
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capacity. To determine the appropriate level
of ramp metering control for a given
freeway, the incremental benefits produced
by local and systemwide traffic-responsive
metering (relative to a base of pretimed
metering) must be estimated. Computer
simulation can be effectively used in
evaluating control benefits. In addition, the
incremental system costs (installation,
operation, maintenance) and the incremental
user costs (travel time, vehicle operating
costs, accidents, air pollution emission) must
be estimated. Incremental benefits and
incremental costs can then be used to
conduct a benefit/cost or utility/cost analysis
to decide upon the most desirable type of
ramp metering.
Making a detailed analysis of freeway
operations and determining improvement
alternatives.
C
Examining the feasibility of ramp control
as an improvement alternative.
C
Analyzing the site conditions and
selecting the control level.
A detailed discussion of the incremental
benefits of different types of ramp metering
control is provided in NCHRP Report
232.(19)
EXIT RAMP CONTROL
Exit ramp control is seldom used as a means
of freeway traffic control because the
opportunities for its effective application are
limited. In many situations, the use of exit
ramp control may actually be contrary to the
objective of safe and efficient freeway
operations. Also, it should only be used
where destinations can easily be reached by
using alternate exits.
The growth in traffic demand over the
lifetime of a ramp metering project may
reduce the incremental benefits of a trafficresponsive type of ramp metering control
(local or systemwide). As traffic demand
grows substantially over the lifetime of the
project, the controllability index of the
freeway decreases. Since the benefits are
nonlinearly related to the controllability, it is
possible that the benefits could decrease
faster than the growth rate in demand. In
planning ramp metering installations, the
engineer should be aware of this effect. It is
recommended that the analysis be repeated
for as many years as are in the expected
project-life duration.
Exit ramp closure can be used effectively to
reduce safety hazards and congestion caused
by excessive weaving between closely
spaced ramps and long queues on exit
ramps. Also, exit ramp closure can be used
at a lane drop location by closing
downstream exit ramps in order to
encourage more traffic to leave the freeway
at the exit ramps before the lane drop and
thus decrease the demand on the freeway
section beyond the lane drop. However, as
in the case of entrance ramp closure, exit
ramp closure might not be acceptable
because of the increased travel it creates for
some motorists.
The incremental benefits analysis is but one
component of a system selection process
which, in turn, is a component of a freeway
traffic management decision process. The
major components of this decision process
include the following:
C
C
Developing a basic analysis of freeway
operations.
EMERGING TECHNOLOGIES
As mentioned in the opening paragraph of
this section, most of the advances and
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deployment horizon.(2) While most of
the
elements
are
hardware
oriented,“processing technology and
advanced algorithms that enable
advanced vehicle and traffic control
application” are designated as mixed,
meaning that there is opportunity for
emerging applications.
emerging
technologies
in
freeway
management systems are in the computing
hardware and communications technologies.
While development of those fields will
continue to enhance the ramp control
process, there will be emergence or at least
advancement, from preliminary stages of
freeway ramp control systems.
Such
advancements include:
C
C
System Operation. As freeway systems
expand with more communications links,
and detector data become available,
there will be increased system operation
of entrance ramp meters, with metering
rates being determined on a system or
subsystem basis.
C
Integrated Systems. Earlier freeway
management systems generally operated
independently of the operation of surface
street signal systems. In lieu of actually
being integrated through hardware and
communications links, traffic state and
local system managers sometimes
communicated informally to bridge the
two systems. Future systems will likely
be fully integrated, with data exchange
and control decisions being made
automatically with real-time data.
C
Information to Motorists. Advanced
information systems being installed or
planned as part of the ITS deployment
and expansion will assist motorists in
selecting or bypassing entrance ramps
where metering rates may be restrictive.
Such diversion can be considered in
integrated freeway and surface street
systems.
C
Advanced Control Algorithms. The
National
ITS
Architecture
Implementation Strategy provides an
evaluation of ITS Technology Areas as
to their maturity (mature, immature,
mixed) to assess their potential
Advanced Ramp Metering Concepts.
Because queues become critical under
heavy ramp demands conditions,
improved queue management algorithms
based on multipoint detection are under
development. Also, traffic responsive
activation of ramp control will likely be
used to manage traffic during off-peak or
weekend incident conditions.(7)
5.4 LESSONS LEARNED
Although ramp control systems have been in
operation in various metropolitan areas
throughout the country for over a quarter of
a century, they are still sometimes viewed as
a “new or radical” approach to traffic
control and management. Intersection traffic
signals, on the other hand, are accepted by
most drivers as necessary and, in fact, their
installation is often requested by citizens.
The two systems essentially perform the
same function: Facilitate use of available
capacity between conflicting vehicular
movements on the basis of demand levels
and safety considerations with traffic
signals. However, the ramp signal may be
viewed negatively by drivers, because
freeways have been traditionally designed for
unrestricted flow. In reality, the flow is
often restricted by recurring and nonrecurring congestion that may have a greater
effect than that of the meter signal, which
may encourage the driver to divert. For
these reasons, there are certain “lessons
learned” associated with ramp control which
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may not be a factor in other traffic and
freeway control elements.
the factors in evaluating and selecting new
metered locations.
IMPLEMENTATION
Public relations aspects of the ramp control
system should begin well in advance of turnon. In Seattle, the Washington State DOT
(WSDOT) has developed a methodical
approach to implementing ramp metering. (30)
Their process describes what needs to be
accomplished starting five years prior to
ramp metering all the way up to one week
before, and continuing through six months
after start-up. The procedure includes public
input, the design process, and the public
relations focus. In Tacoma, Washington, the
WSDOT went beyond the typical public
relations campaign of brochures and media
advertisements. WSDOT has incorporated
a ramp metering lesson into both public and
private driver education school curricula.
The lesson, which lasts about 30 minutes,
helps students to understand what ramp
meters are and what they mean to the driver.
The information packet for this lesson
includes a lesson plan, information sheets,
brochures, key chains, and a well-developed
12 minute video entitled “Ramp Meters:
Signals for Safety.”
Public Relations
Ramp metering systems can be successful
only if they receive public support from
political leaders, enforcement agencies, and
the motoring public. To gain this support in
advance of implementation, a comprehensive
public relations and information program
should begin well in advance. To the public,
ramp meters are often seen as a constraint on
a roadway normally associated with a high
degree of freedom. Although definite
benefits may be achieved by metering and
have been demonstrated statistically, the
benefits may not be recognized by individual
motorists. A 3-minute wait at an entrance
ramp, however, is easily recognized. A
proactive public relations program should be
an integral part of every metering project.(1)
It is important not to oversell the benefits of
ramp metering. It is not a substitute for a
new freeway lane.
The benefits are
measurable systemwide, but may not be
readily discernable to the individual driver at
the ramp signal. Successful public relations
campaigns will explain the difficulties of
mitigating freeway congestion problems and
the cost effectiveness of management
techniques such as ramp metering.(1) The
campaigns should also provide realistic
expectations of the system's benefits, and
show how taxpayers will experience
improved freeway conditions. The most
common method of disseminating ramp
metering information is through brochures or
media advertisements on television and
radio. Some examples of public relations
brochures are shown in reference 1. In
Minneapolis and Los Angeles, the “public”
has actually requested additional metered
ramps. This public input has become one of
A promotional videotape from the FHWA
entitled “Ramp Metering: Signal for
Success” is another example of how the
merits of ramp metering can be presented to
the public.(1) This 17-minute videotape,
which is intended for citizens and public
officials, explains the principles and benefits
of ramp metering. It addresses key issues
such as safety, efficiency, equity, and public
relations. Copies are available through the
FHWA or the Institute of Transportation
Engineers (ITE).
Media Relations
The print and electronic media can be great
allies or great deterrents to the success of
ramp control systems. When the Dallas
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Corridor Study metering system was
implemented in 1974, a radio reporter in the
control center (with CCTV and other
displays) reported that the system was
working great, while a television reporter
interviewing the 20th vehicle in a ramp
queue proclaimed the system a failure.(31)
The system perspective (which was
understood by the reporter in the control
center) must be stressed. As with the
general public, the media must be informed
as to system goals and expectations,
schedules, operations, and results. It is also
important to maintain communication with
the media after system turn-on. Beat
reporters are often reassigned, and the new
reporter may need to be briefed before an
uniformed, negative story is written.
... drivers and their views are
important and a very high priority.
No ramp delays (for a while at least)
will be more than 2 minutes, and
this must be verified. When queues
or delays get too long, the signals
are shut off until the queues clear,
no matter what happens to the
freeway. For the first three months,
metering during the peak of the rush
hour was sometimes terminated. No
written complaints were received.
However,
continuous
quality
improvement for the freeway traffic
flow is stressed. Freeway drivers
have called by cell phone and by
Internet asking TranStar (the
freeway management center) for
“more” ramp metering. Now, the
simple explanation for this is that we
have “teased” the freeway traffic
into this position. But we have not
followed any ramp control strategy
mentioned in the traditional freeway
ramp control manuals.
The
traditional
demand/capacity
methods are for marginally
overloaded
well-disciplined
systems, and that goal of
demand/capacity control is only a
faint vision in Houston at the
moment. We are simply pushing
back up the q/k curve toward
capacity
from
stop-and-go
conditions, and not from the other
side.
Implementation Strategies
Scheduling of ramp control turn-on should
be carefully considered.
Incremental
implementation of individual sections should
be considered, rather than a total system
launch. In particular, locations that have the
best alternate routes and the highest
probability of disruption of traffic flow
should be considered first. Ramps should be
operated with metering rates that cause little
disruption. As drivers become familiar with
and accustomed to the system, metering
rates can be tightened and other locations
implemented.
An interesting approach has recently been
employed in Houston.
Some of the
pioneering efforts in ramp control took place
in the mid-sixties.(12) However, due to
reconstruction of freeways, ramp metering
had not been in operation for some time.
When ramp metering was recently
reimplemented, a conservative philosophy
was developed.
The Implementation
philosophy was as follows: (32)
Implementation Summary
The successful implementation of a freeway
ramp control system is dependent on many
factors outside of the hardware, software,
and control algorithms. The implementation
plan must include involvement, education,
and support by the public, media, and
political leaders. Additionally, the strategy
with which individual ramps and subsystems
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are “turned on” must be carefully
considered, planned, and executed.
C
Evaluation. Although effectiveness of
ramp control techniques has been well
documented in the literature, it is usually
necessary to perform “before and after”
studies to document results of each
system. It will also be important to
continue to sample system operation
with the same type data used in the initial
evaluation to detect changes in system
operation performance.
C
Updating Initial Strategies. Based on
continued system monitoring, as
mentioned above, changes in individual
control parameters or control strategies
may be warranted. These may require
minor changes to the data base or more
significant changes to the control
programs. Changes in the roadway
system, both freeway and surface streets,
must also be monitored and considered.
C
Incorporating New Strategies. As
ramp control systems continue to grow
and mature, new ramp control
algorithms will likely also be developed
and tested. Continued communications
among
system
operators
and
participation
in
professional
organizations such as the Transportation
Research Board (Freeway Operations
Committee), Institute of Transportation
Engineers, and Intelligent Transportation
Society of America will be beneficial in
becoming aware of such strategies.
C
Hardware
and
Software
Maintenance. Hardware maintenance
may be performed either by the agency
or by contract, or by a combination of
the two. The responsible agency will
likely maintain standard traffic control
equipment and communications cables.
Computer and communications hardware
will usually be maintained by contract.
Software data bases will normally be
maintained by the responsible agency,
OPERATIONS AND MAINTENANCE
Operations and maintenance considerations
are not unlike those for other freeway
control subsystems or for other traffic signal
control systems. While the strategies may
differ, there is still a necessity for operating
agencies to commit the funds for personnel
to operate, maintain, evaluate, and update
the control system.
C
Personnel. Adequate personnel for
system operation and maintenance are
essential if systems are going to succeed
and continue to succeed.
While
improved hardware and software
capabilities have allowed many tasks to
be automated in system operation,
personnel must be assigned to ensure
continued efficient operation.
C
Training.
Training for system
operations and maintenance is usually
provided by the systems contractor.
Continuing training programs will be
essential as new personnel are assigned
and as hardware and software upgrades
are implemented.
C
Documentation. Initial documentation
for system operation and maintenance
should (must) be provided by the
systems contractor. Operations and
maintenance personnel must also ensure
that documentation is updated as system
changes or hardware upgrades are made.
Detailed logs should be kept for such
changes.
Modern systems often
incorporate automated logging capability
to facilitate the task and ensure that
records are consistent.
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while applications and system software
will be maintained by contract.
Whatever the method, agency or
contract, maintenance responsibilities
should be clearly defined and understood
in advance of system implementation.
Sufficient funding must continue to be
committed for hardware and software
maintenance.
In Portland, city officials were very
concerned about entrance metering creating
problems on parallel streets. Before the
meters on I-5 were installed, the city and
State agreed that if volumes on adjacent
streets increased by more than 25 percent
during the first year of operation, the State
would either abandon the project or adjust
the meters to reduce the diversion below the
25 percent level.
Following meter
installation, the increase in local street
volume was not substantial. Evaluations of
the impact of metering on adjacent streets
have been conducted in Los Angeles,
Denver, Seattle, Detroit, and other cities.
Significant diversion from the freeway to
surface streets did not occur in any of these
locations. Formal and informal agreements
are common between State and local
jurisdictions in connection with metering
projects, and close advance coordination
between
jurisdictions
is
highly
(1)
recommended.
DIVERSION OF TRAFFIC
A major issue that is raised in connection
with metering is the potential diversion of
freeway trips to adjacent surface streets to
avoid queues at the meters. Extensive
evaluations of existing metering systems
show that adjustments in traffic patterns,
after metering is implemented, take many
forms.(1) However, it is possible to predict
the likely impacts of metering before it is
installed. Factors that enter into the analysis
include trip length, queue length, entry delay,
and especially the availability of alternate
routes. The impact of attractive and efficient
alternate routes can be a key factor in the
effectiveness of a ramp metering system.(33)
The probable new traffic patterns, including
diversion, can then either be accommodated
in the design and operation of the system, or
become part of a decision that metering is
not feasible.
In some cases, there may not be feasible
alternates routes, due to barriers such as
rivers, railroads, or other major highways.
Metering still can and does operate
effectively where diversion is not an
objective of the system. The systems in
Denver, Northern Virginia, and Chicago, for
example, operate under a so-called nondiversionary strategy. In these systems,
metering is sometimes terminated at least
until the queue dissipates. (See discussion of
Houston ramp metering above). Significant
benefits in freeway flow and accident
reduction still result from nondiversionary
metering. The onset of mainline congestion
consistently begins later in the peak period
and ends earlier. On many days, the mainline
does not break down at all. Accidents and
accident rates are also reduced. For
example, in Denver it was observed that
many drivers entered the freeway earlier in
the morning. Peaks or spikes in volumes
were thus leveled out over a longer period of
Metering may, in fact, divert some short
trips from the freeway. In concept, freeways
are not intended to serve very short trips,
and diverting some trips may even be
desirable if there are alternate routes that are
under-utilized. Diverting traffic from high
volume, substandard, or other problem
ramps to more desirable entry points should
be an objective of metering where it is
feasible. Such an action does require a
thorough analysis of the alternate routes and
the impacts of diversion on those routes, and
improvements on the alternate routes when
and where they are needed.
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time resulting in better utilization of freeway
capacity.(34)
metering in Detroit operated only in the
outbound direction to minimize the citysuburb equity problem. Once the
effectiveness of the metering was
established, the system was expanded with
less objection. This strategy was used in
Atlanta where northbound I-75, leaving the
city during the evening peak, will be the first
section metered. (33) In Seattle, the system
was designed to allow more restrictive
metering rates farther away from downtown.
With the long trip length, motorists
originating from the suburbs have the most
to gain from improved freeway conditions.
The minor additional delay experienced at
the meters is more than offset by the reduced
mainline travel times. In Milwaukee, where
the question of equity has been a limiting
factor in the expansion of metering, it is now
proposed to expand the system by metering
each ramp that contributes traffic to
congested freeway segments. Metering rates
will be designed to be comparable for all
ramps. For example, if it is determined a
10 percent reduction in demand is needed on
the freeway segment, metering rates will be
established to reduce all ramp volumes by
10 percent. In addition, each ramp metering
rate will be adjusted to the extent possible in
order to ensure average motorist delays are
about equal for outlying ramps and for closer
in ramps.(35) In Dallas, there was concern
that suburbs were being favored over areas
closer to the central business district. Ramp
counts and license plate studies revealed that
approximately as many vehicles were exiting
the freeway before they reached downtown
as were entering downstream of the adjacent
suburbs, so equity was achieved.(36)
ENFORCEMENT
The effectiveness of ramp metering, like that
of any other traffic regulation, is largely
dependent on voluntary driver compliance.
As part of the public information effort, it
should be made clear that ramp meters are
traffic control devices that must be obeyed.(1)
The laws and penalties should be clearly
explained. In cities where the advance
publicity has been positive and plentiful,
violation rates has been lower. Again, as
with any other regulation, enforcement is
needed. Cooperation with police agencies is
essential. Effective enforcement requires
good enforcement access, a safe area for
citing violators, adequate staff, support by
the courts, and good signs and signals that
are enforceable. Enforcement needs must be
considered and accommodated early in the
project development and design stages.
Enforcement personnel should also be
included early on in the planning and design
of ramp metering projects. Compliance is
critical to the success of a ramp metering
system. Compliance rates, have generally
been good in most areas across the country.
However, violations are contagious and can
multiply quickly. The result can be an
extremely ineffective ramp metering system.
EQUITY
The complaint that ramp metering favors
longer trips at the expense of shorter trips
can be a controversial issue.(1) Close-in
residents argue they are deprived of
immediate access to the freeway, while
suburban commuters can enter beyond the
metered zone and receive all the benefits
without the ramp delays.
Even if only a few drivers experience
increased travel times, there may still be
objections simply because some have to wait
at the ramps and others do not. A
reasonable analogy can be made between a
metered freeway and a signalized arterial.
Vehicles entering an arterial from a minor
Again there are strategies that have been
employed to mitigate the equity issue. Initial
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street must generally wait at a traffic signal
while traffic already on the arterial is given
priority. In both cases, the freeway and the
arterial, the entering vehicles experience
some delay in order to serve the higher
volume facility. (1)
more than 2300, an increase of about 45
percent. Additionally, many existing systems
are proposing expansions and/or upgrades.
On the planning side, new ramp metering is
being considered in numerous other cities as
part of ITS early deployment plans or
feasibility studies. By the year 2000, at least
33 cities in the United States and Canada
will have functioning ramp meters. This will
be 11 more systems than existed in 1989.
5.5 EXAMPLES IN RAMP
CONTROL
There is extensive documentation of ramp
control systems in the literature, much of
which are cited in the reference lists in this
handbook. An excellent summary of ramp
control status, Ramp Metering Status in
North America, was published by FHWA in
1995.(1) The history and case studies cited
below were adapted from that report.
ENTRANCE RAMP METERING
CASE STUDIES
The abbreviated case studies presented here
are just a few examples of effective ramp
metering operations. The benefit statistics
presented are not consistent from city to city
as there is no uniform evaluation criteria.
Additionally, the measures of effectiveness
(MOEs) vary depending on the objectives of
the system. Further, complicating the
matter, many ramp metering installations are
implemented at the same time as other
freeway improvements such as increased
capacity, high-occupancy vehicle (HOV)
lanes,
surveillance
systems,
traffic
information
systems,
and
incident
management programs. In these cases, it is
not always possible to evaluate the individual
components of the larger projects. The
conditions of the evaluations of these case
studies are noted for each discussion.
HISTORY OF RAMP CONTROL
The first metered ramp, as we know it today,
was installed in Chicago on the Eisenhower
Expressway in 1963. This first application,
however, was preceded by successful tests
of the effectiveness of metering traffic
entering New York tunnels, and by ramp
closure studies in Detroit. In Los Angeles,
ramp metering began in 1968. That system
has been expanded continually until there are
now over 800 ramp meters in operation in
L.A. County—the largest system in North
America. Currently ramp meters are in
operation in 23 metropolitan areas in North
America These metering systems vary from
a fixed time operation at a single ramp to
computerized control of every ramp along
many kilometers of a freeway.
Portland, Oregon
The first ramp meters in the Pacific
Northwest were installed along a 10
kilometer section of I-5 in Portland in
January 1981. The meters are operated by
the Oregon Department of Transportation. I5 is the major north/south link, and is an
important commuter route through the
metropolitan area. This initial system
consisted of 16 metered ramps between
downtown Portland and the Washington
state line. Nine of the meters operated in the
Many reports have been written that
document the potential successes and
benefits of ramp metering. However, the
true measure is in the continued growth of
ramp metering installations. Since 1989, the
number of operating meters in North
America has increased from about 1600 to
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northbound direction during the p.m. peak,
and seven controlled southbound entrances
during the a.m. peak. The meters operate in
a fixed time mode. There are currently 58
ramp meters operating on 5 different
freeways.
section of I-35E has been evaluated
periodically since the meters were installed.
The most recent study shows, that after 14
years of operation, average peak hour speeds
remain 16 percent higher, from 60 to 69 ki/h,
than before metering. At the same time, peak
period volumes increased 25 percent due to
increased demand. The average number of
peak period accidents decreased 24 percent,
and the peak period accident rate decreased
by 38 percent.
Prior to metering, it was common along this
section of 1-5 for platoons of vehicles to
merge onto the freeway and aggravate the
already congested traffic. The northbound
PM peak hour average speed was 26 ki/h.
Fourteen months after installation, the
average speed for the same time period was
66 ki/h. Travel time was reduced from 23
minutes (but highly variable) to about 9
minutes. Premetered conditions in the
southbound a.m. peak were much less
severe, hence the improvements were
smaller. Average speeds increased from 64
to 69 kph, resulting in only slight reductions
in southbound travel times.
In 1974, a freeway management project was
activated on a 27-km section of I-35W from
downtown Minneapolis to the southern
suburbs. In addition to 39 ramp meters, the
system included 16 closed-circuit television
(CCTV) cameras, 5 dynamic message signs
(DMS), a 2-km zone of highway advisory
radio (HAR), 380 vehicle detectors, and a
computer control monitor located at the
MnDOT Traffic Management Center in
Minneapolis. This project also included
extensive “freeway flyer” (express bus)
service, and 11 ramp meter bypass ramps for
HOV's. An evaluation of this project after 10
years of operation shows that average peak
period freeway speeds increased from 55 to
74 ki/h, or 35 percent. Over the same 10year span, peak period volumes increased 32
percent, the average number of peak period
accidents declined 27 percent, and the peak
period accident rate declined 38 percent.
Over one million dollars a year in road user
benefits are attributed to reduced accidents
and congestion. This system also has
positive environmental impacts. Peak period
air pollutant emissions, which include carbon
monoxide, hydrocarbons, and nitrogen
oxides, were reduced by just under 2 million
kilograms per year. (38)
Additional benefits that were evaluated for
the p.m. peak period included fuel savings
and a before-and-after accident study. It
was estimated that fuel consumption,
including the additional consumption caused
by ramp delay, was reduced by 2040 liters of
gasoline per weekday. There was also a
reduction in rearend and sideswipe
accidents. Overall, there was a 43 percent
reduction in peak period traffic accidents. (37)
Minneapolis/St. Paul, Minnesota
The Twin Cities Metropolitan Area Freeway
Management System is composed of several
systems and subsystems that have been
implemented over a 25-year period by the
Minnesota Department of Transportation.
The first two fixed time meters were
installed in 1970 on southbound I-35E north
of downtown St. Paul. In November 1971,
these were upgraded to operate on a local
traffic responsive basis and 4 additional
meters were activated. This 8-kilometer
Over 300 additional ramp meters have been
implemented from 1988 to 1995, and there
are currently 400 meters in operation.
Further projects are now in the design and
construction phases. The plans are to
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complete the ramp metering system which
will cover the entire Twin Cities freeway
network over the next five years.(39) The
success of the Twin Cities system has shown
that the staged implementation of a
comprehensive freeway management system
on a segment-by-segment, freeway-byfreeway basis, over a long period of time, is
an effective way of implementing an areawide program.
diversion
through
a
residential
neighborhood. The meters were installed on
the two eastbound ramps on SR-520
between I-5 and Lake Washington. One of
these ramps, the Lake Washington
Boulevard on-ramp, is the last entry onto
SR-520 before the Evergreen Point Floating
Bridge. Because there were no bottlenecks
downstream of this ramp, traffic would
normally flow freely on the bridge and
beyond. Motorists, especially commuters
from downtown Seattle, were using
residential streets to reach the Lake
Washington Boulevard on-ramp to avoid
congestion on SR-520. This on-ramp,
however, was a major contributor to
congestion on SR-520 because of the high
entering volumes. By metering the ramp, it
was anticipated that traffic diverting through
the adjacent neighborhood from downtown
would be discouraged by the delay caused by
the meter. Motorists would instead use the
Montlake Boulevard on-ramp, which was
also metered at the same time. A HOV
bypass lane was also installed at the
Montlake Boulevard on-ramp. Two other
objectives of this project were to improve
flow on SR-520 and to encourage increased
transit use and carpooling.
Seattle, Washington
In September 1981, the Washington State
Department of Transportation (WSDOT)
implemented metering on I-5 north of the
Seattle Central Business District. Initially
the system, which is named FLOW (not an
acronym), included 17 southbound ramps
that were metered during the a.m. peak, and
5 northbound ramps that were metered
during the p.m. peak. Currently, the ramp
metering system includes 54 meters on I-5,
I-90, and SR 520. These meters are all
operated under centralized computer
control. Future expansion plans include
additional ramp meters on SR 520 east of
Lake Washington, on all of I-405, and on I-5
south of Seattle.
One evaluation of the initial 22 meter system
showed that between 1981 and 1987,
mainline volumes during the peak traffic
periods increased 86 percent northbound and
62 percent southbound.
Before the
installation of metering, the travel time on a
specific 11-km course was measured at 22
minutes. In 1987, the travel time for the
same course was measured at 11.5 minutes.
Over the same 6-year time period, the
accident rate decreased by 39 percent.(40)
An evaluation of this two-ramp meter
“system” after four months of operation
showed there was a 6.5 percent increase in
mainline peak period volume, a 43 percent
decrease in the volume on the Lake
Washington Boulevard on-ramp, an 18
percent increase in the volume on the
Montlake Boulevard on-ramp, and a 44
percent increase in HOVs using the
Montlake Boulevard on-ramp.(41) Another
indication of the effectiveness of the
combination of the HOV bypass and the
improved SR-520 flow is a decrease of 3
minutes in METRO (King County
Department of Metropolitan Services) transit
travel times for buses traveling from
downtown to the east, and a 4-minute
A somewhat unique application of metering
was implemented in Seattle on SR-520 in
1986. While diversion caused by metering is
often controversial, one of the objectives of
metering SR-520 was to reduce commuter
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decrease for buses traveling from University
District to the east. The reliability of the bus
travel times also improved, and METRO
adjusted the schedules for these routes
accordingly.
The success of the pilot project led to
expansion of the system. In 1984, a central
computer was installed and a System
Coordination Plan was implemented that
permits central monitoring and control of all
meters. Since 1984, additional ramp meters
have been added, until reaching the current
number of 28. In late 1988 and early 1989,
a comprehensive evaluation of the original
metered section was conducted. A number
of changes occurred between 1981 and
1989, the most significant of which was the
completion of a new freeway, C-470, which
permitted more direct access to I-25 from
the southwest area and generated higher
demand for I-25. Volumes during the 2hour a.m. peak period increased from 6200
vph in 1981 to 7350 vph in 1989 (on 3
lanes). Speeds measured in late 1988
decreased from the original evaluation, but
remained higher than the speeds before
metering was implemented: 69 ki/h before,
85 ki/h after, in 1981, and 80 ki/h in late
1988. The frequency of accidents during the
a.m. peak period did not increase between
the time of original evaluation and 1989. As
a result, the accident rate decreased
significantly because of the increased
volumes. Rearend and sideswipe type
accidents decreased by 50 percent during
metered periods.
In 1993, the WSDOT implemented weekend
ramp metering for the first time. Three
ramps north of Seattle on southbound I-5
have been metered several hours due to
heavy weekend volumes. Because of this
success, in March of 1995, weekend
metering was expanded to include four
additional southbound ramps.
In April of 1995, WSDOT began operating
seven southbound I-5 meters during the
evening commute. This is WSDOT’s first
implementation of metering both directions
of a corridor during the same peak period.
The motivation behind this operational
change is that the traditional reverse
commute direction has become increasingly
congested. Prior to this change, metering
along this section had operated southbound
(inbound toward Seattle) during the morning
commute and northbound (outbound) during
the evening commute.
Denver, Colorado
The Colorado Department of Transportation
activated a pilot project to demonstrate the
effectiveness of ramp metering on a section
of northbound I-25 in March 1981. The
initial system consisted of five local trafficresponsive metered ramps operated during
the a.m. peak on a 4.7-km section of I-25
south of the city. Periodic after-evaluations
revealed significant benefits. An 18-month
after study showed that average peak period
driving speed increased 57 percent and
average travel times decreased 37 percent.
In addition, incidence of rearend and sideswipe accidents declined 5 percent due to
the elimination of stop-and-go conditions.
An interesting unplanned “evaluation” of the
system occurred in the Spring of 1987. To
accommodate daylight savings time, all of
the individual ramp controllers were adjusted
one hour ahead. Unfortunately, the central
computer clock was overlooked. The
central computer overrode the local
controllers, and metering began an hour late.
Traffic was the worst it had been in years.
However, this oversight did have a bright
side for the Department of Transportation.
Since this incident, the media has been even
more supportive of ramp metering than
before. (34)
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In 1988, the Colorado Department of
Transportation conducted a study to
evaluate different levels of ramp metering
control. The study compared ramp meters
operating in local traffic-responsive mode
versus meters operating under centralized
computer control. The results showed that
if local traffic-responsive metering could
maintain freeway speeds above 90 ki/h,
centralized control offered little or no
additional benefit. However, if local trafficresponsive metering was unable to maintain
speeds near the posted speed limit of 90 ki/h,
centralized control was very effective. Data
showed speeds increased 35.5 percent,
from 50 to 68 ki/h, and vehicle hours of
travel were reduced by 13.1 percent.(42)
This evaluation shows the importance of
implementing operating strategies that
correspond to the needs of the freeway
network.
freeway-to-freeway connectors on this
section of I-94.(44)
Austin, Texas
In the late 1970s, in Austin, the Texas
Department of Transportation implemented
traffic responsive meters at 3 ramps along a
4.2 km segment of northbound I-35 for
operation during the a.m. peak period. This
section of freeway had two bottleneck
locations that were reducing the quality of
travel. One was a reduction from 3 to 2
lanes and the other was a high volume
entrance ramp just downstream of a lane
drop. Metering resulted in an increased
vehicle throughout of 7.9 percent and an
increase in average peak period mainline
speeds of 60 percent through the section.
The meters were removed after the
reconstruction of I-35 eliminated the lane
drop in this section. (44) This situation shows
the versatility of ramp metering in that it can
also be used effectively as a temporary
solution.
Detroit, Michigan
Ramp metering is an important aspect of the
Michigan Department of Transportation's
(MDOT) Surveillance Control and Driver
Information (SCANDI) System in Detroit.
The SCANDI metering operation began in
November 1982 with six ramps on the
eastbound Ford Freeway (I-94). Nineteen
more ramps were added on I-94 in January
1984 and three more in November 1985. An
evaluation performed by Michigan State
University for MDOT determined that ramp
metering increased speeds on I-94 by about
8 percent. At the same time, the typical
peak hour volume on the three eastbound
lanes increased to 6400 vehicles per hour
from an average of 5600 VPH before
metering. In addition, the total number of
accidents was reduced nearly 50 percent,
and injury accidents came down 71 percent.
The evaluation done by Michigan State also
showed that significant additional benefits
could be achieved by metering the three
Long Island, New York
At the other end of the spectrum from
Austin is the INFORM (Information For
Motorists) project on Long Island. The
INFORM project covers a 64-km long by
8-km wide corridor at the center of which is
the Long Island Expressway (LIE). Also
included in the system is an east-west
parkway, an east-west arterial and several
crossing arterials and parkways, a total of
207 kilometers of roadways.
System
elements include 70 metered ramps on the
LIE and the Northern State/Grand Central
Parkway.
In 1989, an analysis of the initial metered
segment was conducted after 2 months of
operation. For the peak period, the study
showed a 20 percent decrease in mainline
travel time (from 26 to 21 minutes) and a 16
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percent increase in average speed (from 47
to 56 ki/h). Motorists entering at metered
ramps also experienced an overall travel time
reduction of 13.1 percent and an increase in
average speed from 37 to 45 ki/h. The
MOEs for this project include vehicle
emissions. For this initial segment, the
analysis indicates there was a 6.7 percent
reduction in fuel consumption, a 17.4
percent reduction in carbon monoxide
emissions, a 13.1 percent reduction in
hydrocarbons, and a 2.4 percent increase in
nitrous oxide emissions.
The last is
associated with the higher speeds. Initial
observations of the effect of metering the 4lane parkway on the INFORM project
indicates the benefits may be even greater
than those achieved on wider freeways.
Intuitively this makes sense, because the
impact of an unrestricted merge on only two
lanes (in one direction) can be severe. (45)
period showed an improvement of 25
percent in the congestion index. The
accident frequency rate also showed
encouraging improvement, with a 15 percent
reduction as compared to the control
section. (46)
San Diego, California
In San Diego, ramp metering was initiated in
1968. That system, installed and operated
by the California Department of
Transportation (Caltrans), now includes 134
metered ramps on 110 plus kilometers of
freeway.
No detailed evaluations of
metering have been conducted on the San
Diego system since the early installations,
but sustained volumes of 2200 vph to
2400 vph, and occasionally even higher, are
common on San Diego metered freeways. A
noteworthy aspect of the program is the
metering of eight freeway-to-freeway
connector ramps. Metering freeway-tofreeway connectors requires careful attention
to storage space, advanced warning, and
sight distance. If conditions allow, freeway
connector metering can be just as safe and
effective as other ramp metering. (47)
A more extensive evaluation of the
INFORM project was completed in 1991.
Data from this study showed much more
conservative results. It is believed that this
study is more representative of the true
traffic conditions. The main reason for this
is related to the “queuing off” (shut-down of
the meter due to excessive queuing) of the
ramp meters. The original study did not
include areas where metering was usually
shut off due to heavy ramp volumes, while
the later study accounted for all ramps. This
evaluation showed that while throughput had
increased only about 2 percent, the average
mainline speeds had increased from 64 to 71
ki/h, or about 9 percent. However, for two
separate bottleneck locations, data showed
increases of 53 to 84 and 53 to 89 ki/h, or
gains of about 36 and 40 percent
respectively. This evaluation also included
calculation of a “congestion index.” This
index is the proportion of detector zones for
which speeds were less than 48 ki/h (30
mi/h). While no benefit was shown in the
evening peak period, the morning peak
SUMMARY OF RAMP METERING
BENEFITS
Metering entrance ramps can significantly
improve mainline traffic flow. These case
study evaluations, as well as others, show
that metering consistently increases travel
speeds and improves travel time reliability,
both of which are measures of reduced stopand-go, erratic flow.
It should be
emphasized that these benefits occurred even
though, in most instances, mainline volumes
had significantly increased. Metering helps
smooth out peak demands that would
otherwise cause the mainline flow to
breakdown. A strong case can be made
from the data reported that metering actually
increases the throughput of a freeway. The
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data from Minneapolis, San Diego, Seattle,
Detroit and Denver shows mainline volumes
well in excess of 2100 vph per lane on
metered sections, and sustained volumes in
the range of 5 percent to 6 percent greater
than for pre-metered conditions. Improved
traffic flow, particularly the reduction in
stop-and-go conditions, also reduces certain
vehicle emissions. This has been shown in
both the INFORM project and in the Twin
Cities Freeway Management System.
increased by 12 percent. During the first
three years of metering, total weekday (24
hour/day) crashes increased by 8 percent
while accidents during ramp metering
decreased by 18 percent. The other case
studies presented in this report consistently
show a reduction in crash rates of 24 to 50
percent.
Minnesota Department of
Transportation estimates over 1000 vehicle
crashes are prevented each year on
Minneapolis/St. Paul metropolitan area
freeway due to ramp metering.39 However,
the benefits derived from accident reduction
go well beyond the direct costs related to
medical expenses and vehicle damage. To
illustrate, assume an incident blocks one lane
of three at the beginning of the peak period
on a freeway with a 2-hour peak demand of
6000 vph. Studies show that an accident
blocking one of three lanes reduces capacity
by 50 percent. A 20-minute blockage would
cause 2100 vehicle-hours of delay and a
queue over 3 kilometers long, and take 2 1/2
hours to return to normal, assuming there
were no secondary accidents or incidents.
Clearly the safety aspects of metering are a
major benefit.
The other direct benefit, but one that has not
been fully quantified, is the reduction in
accidents attributed to metering. The Dallas
corridor provided a unique opportunity to
compare vehicle crash experience in a ramp
metering system.(48) Evaluation studies
showed significant improvements in system
operating characteristics as compared to the
“before” conditions. However, during the
first year of operation, metering was
exercised only in the peak direction of flow.
During that year, crashes in the metered
direction decreased by 24 percent as
compared to the previous year, while
crashes in the non-metered direction
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5.6 REFERENCES
1.
Piotrowicz, G., and Robinson. J.R. Ramp Metering Status in North America. Report DOTT-95-17. FHWA, U.S. Department of Transportation, Washington, DC, 1995.
2.
Highway Capacity Manual. Highway Research Board Special Report No. 209, Third
Edition, TRB, National Research Council, Washington, DC, 1994.
3.
Manual on Uniform Traffic Control Devices. FHWA, U.S. Department of Transportation,
Washington, DC, 1988.
4.
ITS Architecture Summary. Joint Architecture Team, FHWA, U.S. Department of
Transportation, Washington, DC, 1996.
5.
Sumner R., et. al. Freeway Management Handbook, FHWA, U.S. Department of
Transportation, Washington, DC, 1983. (Four volumes.)
6.
Wilshire, R., et al. Traffic Control Systems Handbook - Revised Edition - 1985, FHWA1P-85-11, FHWA U.S. Department of Transportation, Washington, DC, 1985.
7.
Gordon, R., et. al. Traffic Control System Handbook - Revised Edition - 1996, FHWA-SA95-032, FHWA, U.S. Department of Transportation, Washington, DC, 1996.
8.
Hanshin Expressway Traffic Control System. Osaka Management Division, Hanshin
Expressway Public Corporation, Osaka, Japan, March 1973.
9.
Traffic Control of Metropolitan Expressway, Metropolitan Expressway Public Corporation,
Tokyo, Japan, May 1973.
10. Wattleworth, J.A., Courage, K.G., and Carvell, J.D. An Evaluation of Two Types of
Freeway Control Systems. Report 488-6, Texas Transportation Institute, Texas A&M
University System, College Station, TX, 1968.
11. Newman, L., Dunnet, A., and Meis, G. Freeway Ramp Control - What It Can and Cannot
Do. Freeway Operation Department, District 7, California Division of Highways, February
1969.
12. McCasland, W.R. Freeway Ramp Control System. Report 24-26F, Texas Transportation
Institute, Texas A&M University System, College Station, TX, 1969.
13. Institute of Transportation Engineers, Technical Committee 4M-11. “Displays for Metered
Freeway Entrance Ramps.” ITE Journal, Vol. 54, No. 4, Washington, DC, April 1984, pp
14-18.
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14. Masher, D. P., et al. “Guidelines for Design and Operation of Ramp Control Systems.”
NCHRP Project 3-22 Final Report, December 1975. (Loan copy available on request to NCHRP.)
15. Courage, K. “Freeway Surveillance and Control: A State of the Art Summary.” Paper
presented at the Computer Traffic Control Workshop. University of Minnesota, March 2022, 1974.
16. Drake, J.S., Schofer, J.L., and May, Jr., A.D. “A Statistical Analysis of Speed Density
Hypotheses.” Highway Research Record 154, HRB, National Research Council,
Washington, DC, 1967, pp 53-87.
17. Breiman, L. “A Case Study - LAAFSCP.” Paper presented at the Urban Freeway
Surveillance and Control Short Course. Technology Service Corporation, Bethesda,
Maryland, March 1974.
18. McDermott, J.M., Kolenko, J.J., and Wojcik, R.J. Chicago Area Expressway Surveillance
and Control: Final Report. Expressway Surveillance Report No. 27. Illinois Department of
Transportation, Springfield, IL, March 1979.
19. Blumentritt, C. W., et al. “Guidelines for Selection of Ramp Control Systems.” NCHRP
Report 232, Washington, DC, May 1981.
20. Carvell, J.D. Freeway Operations Manual. Dallas Corridor Study. Research Report 95311, Texas Transportation Institute, Texas A&M University System, College Station, TX,
1974.
21. Drew, D.R., LaMotte, L.R., Wattleworth, J.A., and Buhr, J.H. “Gap Acceptance in the
Freeway Merging Process.” Highway Research Record 279, HRB, National Research
Council, Washington, DC, 1969, pp 137-149.
22. Buhr, J.H., McCasland, W.R., Carvell, J.D., and Drew, D.R. “Design of Freeway Entrance
Ramp Merging Control Systems.” Highway Research Record 279, HRB, National Research
Council, Washington, DC, 1969. pp 137-149.
23. Drew, D., McCasland, W.R., Pinnell, C., and Wattleworth, J.A. The Development of An
Automatic Freeway Merging Control System. Research Report 24-19. Texas Transportation
Institute, Texas A&M University System, College Station, TX, 1966.
24. Wattleworth, J.A., and Berry, D.S. “Peak-Period Control of a Freeway System - Some
Theoretical Investigations.” Highway Research Record 89, HRB, National Research
Council, Washington, DC, 1965, pp 15-20.
25. May, A.D., Jr. “Peak-Period Analysis and Control of a Freeway System.” Highway Research
Record 157, HRB, National Research Council, Washington, DC, 1967, pp 15-20.
5-60
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26. Courage, K.F., and Levin, M. A Freeway Corridor Surveillance Information and Control
System. Research Report 438-8, Texas Transportation Institute, Texas A&M University
System, College Station, TX, December 1968.
27. Messer, C. J. “Development and Evaluation of a Multi-Level Freeway Control System for
the Gulf Freeway.” Paper presented to the Freeway Operations Committee, 50th Annual
Meeting of the Highway Research Board, Washington, DC, 1971.
28. Looze, D.P., et al. “On Decentralized Estimation and Control with Application to Freeway
Ramp Metering.” IEEE Transactions on Automatic Control, Vol. AC-23, No. 2, April 1978,
pp. 268-275.
29. Messer, C.J., et al. Ramp Adaptive Metering Bottleneck Optimization (RAMBO) User’s
Manual. Research Report 1232-31, Texas Transportation Institute, College Station,
November 1994.
30. TSMC SC&DI Operations/Implementations Plan. Washington State DOT, October 1994.
31. Carvell, J.D. Dallas Corridor Study-Final Report, Report 953-29, Texas Transportation
Institute, College Station, TX, August , 1976.
32. Personal Correspondence with Dr. Carroll Messer, Research Engineer, Texas Transportation
Institute, College Station, TX, November 1996.
33. Yagar, S., “Predicting Impacts of Freeway Ramp Metering on Local Street Flows and
Queues”, ITE 1989 Compendium of Technical Papers, September 1989.
34. Corcoran, L.J., and Hickman, G.A. “Freeway Metering Effects in Denver.” ITE 1989
Compendium of Papers, September 1989.
35. Recommended Ramp Metering Strategy for the Atlanta Regional Advanced Transportation
Management System, NET/TRW for the Georgia Department of Transportation, September,
1993.
36. A Freeway Management System Plan for the Milwaukee Area. Southeastern Wisconsin
Regional planning Commission, November 1988.
37. I-5 North Freeway Ramp Metering, Portland, Oregon: Project Development Operation,
Oregon Department of Transportation-Metropolitan Branch, June 1982.
38. Freeway Traffic Management Program: Status Report, Minnesota Department of
Transportation, January 1989.
39. Overview of the Minnesota DOT Ramp Metering Program, Minnesota DOT, 1995.
40. Henry, K.C., and Mehyar, O. Six-Year FLOW Evaluation, Washington State Department
of Transportation, District 1, January 1989.
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41. Evaluation of the Eastbound SR-520 Traffic Management System, Washington State
Department of Transportation, no date.
42. Lipp, L.E., Corcoran, L.J., and Hickman, G.A. “The Benefits of Central Computer Control
for the Denver Ramp Metering System.” TRB Annual Meeting, January 1991.
43. Maleck, T.L., Kostyniuk, L.P., Taylor, W.C., and Hamad, A.I. An Evaluation of the
Detroit Freeway Operations (SCANDI) Project, Michigan State University, June 1988.
44. Marsden, B. Ramp Meters and Travel Quality in Austin, Texas, Texas DH&PT, May 1981.
45. Before and After Study, Ramp Metering, Eastbound Long Island Expressway, Suffolk
County, Technical Memo., James H. Kell & Assoc., April 28, 1989.
46. INFORM Evaluation, Volume 1: Technical Report , FHWA-RD-91-075, January 1992.
47. Harvey, S. “Ramp Meters-What They Can and Cannot Do.” District 6 Newsletter, JulyAugust 1985.
48. Evaluation of Ramp Control Operation: Dallas Corridor Study, Report 836-1, Texas
Transportation Institute, College Station, TX, August 1972.
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