Traffic Control Devices, Visibility, and Railroad Grade Crossings

Traffic Control Devices, Visibility, and Railroad Grade Crossings
Effective Use of Variable Message Signs:
Lessons Learned Through Development of
Users' Manuals
In an effort to improve the operations of both portable and permanent
(fixed-site) variable message signs (VMSs) in Virginia, a comprehensive research effort to develop operational guidelines was undertaken.
These guidelines, presented in the form of users' manuals, were based
on information obtained from the literature, VMS operators, and
motorists. Issues addressed by the manuals include whether to use a
VMS, where to place a portable VMS, and how to design a VMS message. The manuals are not simply a list of predefined messages; instead,
they are composed of concise, readable modules designed to guide an
operator through the thought process required to use a VMS effectively.
An operator follows a logical decision tree as each module is completed,
allowing effective use of the VMS as well as training the operator for
use of the device. Key lessons learned in developing two such manuals
for portable and permanent VMSs are highlighted. On the basis of theoretical calculations and motorists' experiences, it is strongly recommended that a VMS use no more than two message screens. A single
message screen is preferred. VMSs should be used only to advise
drivers of changed traffic conditions and to convey specific traffic information concisely. Because of limited information capabilities, VMSs
should be used in conjunction with other means of communication such
as highway advisory radio and static signs. Most importantly, it is crucial that credibility be maintained. Incorrect information can have disastrous consequences on VMS effectiveness.
The need to provide drivers with real-time information has spawned
a dramatic increase in the use of variable message signs (VMSs).
VMSs are programmable traffic control devices that display messages composed of letters, symbols, or both, and may be either permanently mounted or portable units. The VMS aliows transportation officials to quickly inform motorists of abnormal traffic
conditions. Although the ability of VMSs to display messages that
describe current traffic conditions has made the signs popular, this
added flexibility results in increased operational responsibility to
ensure that the signs are used to their maximum benefit. Some
VMSs are difficult to understand because word choices are confusing, messages contain too much information or are ambiguous, or
placement of portable VMSs is poor.
This research sought to develop user's manuals that provide Virginia Department of Transportation (VDOT) field personnel sufficient detail to effectively use VMSs on the basis of the type of situation, predominant travel speed, time of day, goal of message, and
type of sign (1). These manuals address questions such as
• Under what circumstances should VMSs be utilized?
• Where should a portable VMS be placed?
Virginia Transportation Research Council, 530 Edgemont Road, Charlottesville, Va. 22903.
• What are the limits of the quantity of information that can be
• What information should be given to drivers?
Although a significant amount of VMS research has been conducted nationally, most of it has dealt with physical specifications
of the signs rather than operational issues. The focal question of this
research has been, Given the existing technology, how should
VMSs be operated?
Three information-seeking strategies were used for this project: a
literature search, surveys of VDOT personnel, and discussions with
drivers in the Commonwealth of Virginia. The literature review
provided the base of the study, as a significant amount of VMS
research had been conducted and needed to be assimilated into a
concise format (2-6). Additional ideas were gleaned from other
states' VMS guidelines (7-11). VDOT personnel played an important role because they are the primary users of the manuals, and their
participation ensured the development of a product that meets their
needs. Finally, motorists' reactions provided recommendations for
improvements that make VMSs more useful to the driving population, the ultimate "customer" of traffic information.
The literature addressed the theoretical operation of VMSs, which
provided a solid basis from which to construct the manuals. For
example, one FHWA publication defined the components of an
advisory message to be (a) a problem statement, (b) an effect statement, (c) an attention statement, and (d) an action statement. Such
message deconstruction proved useful for understanding how to
develop effective VMS messages and for forming the message
design modules of the manuals. VMS usage was also addressed. For
example, static signs should be used to complement the VMS and
should be considered before the decision to use a VMS is made.
Working with VDOT VMS operators, it became clear that
previous research was not in an easily used form.
The outcome of the literature review, focus groups, and discussions and surveys involving VMS operators yielded six key
1. VMS operators need a user's manual-not a set of canned
messages. Operators need final responsibility for how the VMS is
used as well as the ability to respond to unforeseen applications. A
list of suggested messages is ineffective, as variations in traffic conditions and available information are numerous. Clearly the operators need a product that is not overly restrictive but is much more
substantive than simply authorizing the use of "engineering judgment." The product needs to take the form of a user's manual, which
will assist operators but not replace their judgment.
2. VMSs should be used to advise the motorist of changed traffic conditions. Operators' field experience showed that VMSs
should be used only to convey information about traffic abnormalities such as lane closures, delays, or sudden stoppages. VMS operators noted that motorists ignored greetings and general safety statements (e.g., "Please drive safely").
3. VMSs must meet motorist information needs. Specifically, the
VMS should tell motorists what action is required of them. Messages such as "LANE CLOSED AHEAD" need to indicate which
lane is being closed and the distance to that closure. General messages such as "SLOW SLOW SLOW" or "CAUTION" are useless,
as they do not inform motorists about traffic conditions. Finally,
word choice has a powerful impact. Motorists noted that the word
"DETOUR" meant static signs would guide them along the alternate route, whereas the phrase "ALTERNATE ROUTE" means
they must find that route on their own.
4. VMSs have limited information capabilities. VMSs can provide effective alternative route guidance for unfamiliar drivers only
if used in conjunction with another information medium, such as
static signs or highway advisory radio.
5. Credibility is crucial. Failure to confirm the message displayed
by a VMS can have disastrous consequences in terms of the public's
faith in future VMS messages. Motorists recalled instances in which
the information was clearly wrong, such as warnings of construction
activity at night when no construction was taking place.
6. VMSs should use no more than two message screens. Even
though portable VMSs may display up to six different message
screens, it is difficult at high speeds 88.6 kph (55 mph) for motorists
to read a message with only two screens.
Motorists cited difficulties reading multiple screen messages
because large vehicles blocked the line of sight, visibility conditions
were poor, there were other distractions, or the sign was placed on
the opposite shoulder. Messages longer than two screens can easily
confuse motorists if they encounter such a VMS in midmessage,
and VMS operators need to allow for this possibility.
Dudek' s technique for determining the amount of time motorists
have to read a VMS demonstrates that a portable VMS should
employ only one or at most two message screens (3). For example,
a portable three-line, eight-character-per line, flip-disk VMS will
cease to be comfortably readable when the motorist gets very close
to it. An equation to account for this distance from the VMS to the
point at which it becomes unreadable is given as
Unreadable Distance = [S
+ (N -
+ 0.5*W]*5.67
N =
L =
W =
distance from the side of the road to the VMS [m (ft)],
number of lanes,
width of the lanes [m (ft)], and
width of the VMS [m (ft)].
This unreadable distance may then be subtracted from the legibility distance, which is the distance at which the VMS becomes
legible, to yield the distance for which the VMS may be read by the
motorist. This calculation is shown in Equation 2.
Readable Distance = Legibility Distance
- Unreadable Distance
The resultant readable distance may then be divided by the travel
speed to compute the time for which the VMS is readable, as shown
in Equation 3.
Readable Time =
Readable Distance
Travel Speed
For example, suppose a VMS is mounted such that, as calculated
by Equation 1, it has an unreadable distance of 61 m (200 ft). The
literature states that in daylight conditions, a flip-disk VMS has a
legibility distance of approximately 198 m (650 ft) (2; P. Garvey,
unpublished data). Therefore, from Equation 2, one may compute
the readable distance to be
198 m - 61 m = 137 m (450 ft)
Substitution of this value and a travel speed of 88.6 kph (55 mph)
into Equation 3 yields a readable time of about 6 sec.
Manual on Uniform Traffic Control Devices (MUTCD)proposed guidelines specify that motorists must be able to read the
entire VMS twice while traveling at the posted speed (12). Using
Dudek's approximation that motorists need 1 sec to read each eightcharacter line, it will take 3 sec to read a screen once or 6 sec to read
a screen twice. Thus for this particular example, an operator should
ideally use a message with only one screen. (Even if one decides to
display each screen for only 1.5 sec these computations show that
no more than two message screens should be used.)
The most challenging task of this project was synthesizing the
results into easy-to-read operator's manuals. The features of these
manuals are discussed.
Use of Modules
The manuals were divided into separate modules designed to step
an operator through the thought process involved in using a VMS
(Figures 1 and 2). A module is a distinct thought process in the overall VMS message development. Each module serves as a checkpoint for ensuring that the correct decisions have been made, such
as whether to use or where to place the VMS.
Using a logical flow of decision points, which is accomplished by
dividing the manuals into modules, ensures that the VMS is used
correctly. For example, one problem that operators often face is how
to convey location information to motorists. Depending on the type
of route, driver familiarity with the area, and amount of signing, it
may be better to tell motorists that there is an accident at a particular exit near a well-known landmark or a certain number of miles
away. If an operator is faced with this decision, Module 10, as indicated in Figure 2, quickly guides the operator to the correct usage.
Furthermore, a progression of decision points allows an inexperienced operator to become familiar with the choices that should be
Miller et al.
A VMS is a tool for grabbing the motorist's attention. Therefore it should be used only when there is a
sped.fie message that needs to be conveyed to motorists: overuse of a VMS will cause motorists to ignore it and
lessen its effectiveness.
&l of the following statements should be true if a VMS is to be used.
Drivers are required to J!Q something in response to the message such as:
change travel speed,
change lanes,
divert to a different route, or,
be aware of a change in traffic conditions either now or in the future.
Where applicable, static signs which can effectively convey this message are not readily
VMSs should be used to supplement, rather than replace, static signs that are required by law.
The VMS does not tell drivers something they already know.
Message accuracy can be confirmed from a reliable source such as State Police, a credible
commercial traffic reporter, or visual inspection.
Traffic conditions may be monitored to detect significant changes such that the VMS may be
removed or the message may be changed as soon as necessary.
FIGURE 1 Module 1.
made when using a VMS, thereby providing a training opportunity
parallel with a VMS operation.
It is believed that the use of modules offers distinct advantages.
First, modules facilitate the updating of the manuals, which must
occur if they are to become and remain a useful tool. Second, modules streamline the VMS decision process: operators need only
complete those modules that are necessary. Often, a decision in one
module will eliminate the need to go through certain subsequent
modules. Finally, modules present the information in a concise,
user-friendly manner.
The example modules, shown in Figures 1 and 2, illustrate the
diversity of input requirements and purposes for the modules. For
example, the first module's purpose is to determine whether a VMS
should be used; thus it should always be completed, and if successful then the result is to simply continue with the second module.
However, the tenth module should be completed only if the operator needs to convey a distance or a location, and the tenth module's
result is a recommended choice of words.
Integration of Modules: An Example Application
The portable VMS manual is divided into 17 modules, as shown in
Figure 3, with each module designed to help the operator answer
one basic question: "Where should the VMS be located?" Figure 3
outlines the purposes and some considerations of each module as
well as how the modules are integrated to guide the operator
through the VMS decision process.
An example scenario briefly illustrates how the operator moves
through these modules. Suppose an operator receives notification
from the supervisor that a truck has crashed on a rural two-lane
Interstate highway, resulting in a traffic queue and blockage of the
left lane, although traffic can pass in the right lane. The goal of the
first module (Figure 1) is to help the operator decide whether to use
the VMS. In this case, the operator makes the decision to use the
VMS; drivers are required to do something (change lanes); static
signs that can inform motorists of an accident are not available; the
VMS conveys new information to drivers (to merge right); message
accuracy can be confirmed (by the supervisor); and the VMS operator will be on the scene to monitor traffic conditions. In the second
module, the operator decides that the purpose of the VMS will be
"current incident advisory." The operator then proceeds to the third
module to determine the location of the VMS, considering factors
such as access to the VMS, major decision points (in this case, Interstate exits), and the effect of future traffic backups. The VMS is thus
placed upstream of an Interstate exit before the crash, and the VMS
is placed off the shoulder in conjunction with Group II channelization devices (e.g., orange barrels) such that the VMS itself is not a
When using a location or distance, the question arises as to whether the operator should give a distance
("ACCIDENT IN 3 MI"), an exit number ("ACCIDENT AFfER EX 100"), or the name of a prominent
landmark ("ACCIDENT AT BROAD STREET"). In order to make this decision, complete this module.
The message applies ONLY to familiar drivers.
The route is an interstate.
The exits are numbered sequentially (i.e. they do not correspond to mile markers).
F At least one of the following are located within one mile after the VMS:
an exit, or
a static sign indicating a distance to an exit
FIGURE 2 Module 10.
safety hazard. Had this been an urban area, the operator would have
considered moving the VMS even further from the crash depending
on previous experience with traffic congestion. In the fourth module, with traffic flowing past at 55 mph (88.6 kph), the operator
decides to try and keep the message to one screen. Module 5 shows
that because this is a "current incident advisory" type of usage, the
operator should complete Module 6, which is the design of the current incident advisory message. In Module 6 the message is synthesized on the basis of the three components of a current incident
advisory message: the problem component (an accident blocking
the left lane), the location component [the incident is about 3 mi (4.9
km) away], and the instruction component (motorists should move
out of the left lane and into the right lane or exit from the Interstate
altogether). The operator tentatively envisions a message such as
"accident-left lane closed-3 mi ahead-merge right." Note that
this module discourages peripheral information, such as a description of the crash (e~g., "truck overturned"); instead, the goal is to
establish the essential message elements.
The operator is now sent to Module 10, which is shown in Figure
2. Because the message applies to drivers both familiar and
unfamiliar with the route and it is an Interstate, the operator arrives
at the third true/false statement shown in that module. If the
exit numbers were according to mile marker, then the operator
would change the phrase "3 mi ahead" to refer to an exit, such as
"after exit 100." In this case, however, suppose exit numbers are
sequential. Because there are travelers using the Interstate who
might not be familiar with the distances between exits, Module 10
advises the operator to give a distance in miles and then proceed to
Module 11. In Module 11, the operator realizes the word "ahead"
is not necessary in the phrase "3 mi ahead." Module 12 is not
applicable as the word "next" has been avoided in the message, so
the operator proceeds to Modules 13 and 14, where the message is
divided into multiple screens, abbreviations are considered, and the
necessity of each screen is scrutinized. Supposing a three-line,
eight-character-per-line VMS, the operator's first attempt might be
as follows:
screen 1
screen 2
The operator then uses abbreviations and develops the following
screen 1
screen 2
What operator accomplishes in this module
Some factors considered in completing the module
1. Should a VMS be used?
If successful, then goes to module 2.
If unsuccessful, then stops.
Change in driver response required, ability of operator to confirm and
maintain message accuracy, and inability of static signs to accomplish
the task.
2. What is the purpose of the VMS?
Selects ONE of these four categories:
(a) current incident or work zone advisory
(b) diversion to an alternate route
(c) guidance for a current special event.
(d) advisory for a future event
Expected purpose of message.
3. Where should the VMS be located?
Determines an acceptable location with respect
to the roadway and the condition bein$ conveyed.
Roadway geometry, presence of major decision points, access to the
VMS, sight distance, and future traffic backups.
4. What is the maximum number of
Conveys to operator that 1 screen is ideal, two
screens are acceptable, and 3 screens should be
used only if absolutely necessary.
Driver inattention, MUTCD guidelines, amount of time required to
read a VMS, legibility distance of a flip-disk VMS, and calculations
detailed in Appendix A.
5. What is the message type?
Selects ONE of the following modules to
complete: 6, 7, 8, or 9.
Category identified in module 2.
6. What is the message?
(current incident or work zone advisory)
Designs a message considering problem, location,
and instruction components.
Example message components, effect of sensationalist messages, and
need to convey specific information to the motorist.
7. What is the message?
(route diversion)
Designs a message considering audience, time
saved, and instruction components.
Example message components, distinction between "alternate route"
and "detour", and the need for "time saved" statement to be accurate
or avoided.
8. What is the message?
(guidance for a special event)
Designs a message considering audience and
instruction components.
Example message components and names or events that motorists will
9. What is the message?
(advisory for a future event)
Designs a message considering condition &
location, time, and instruction components.
Example message components and time requirements (such as not
describing future conditions more than one week in advance).
10. How should distances and locations be
If locations or distances are needed, decides
whether to use landmarks, exit numbers, or
distances in the message.
Type of exit numbering scheme (sequential or by mile marker),
familiarity of ·the driver population, and proximity of static signs.
screens that may be used?
Overview of portable VMS modules.
(continued on next page)
11. Are the words "TRAFFIC" and
"AHEAD" used correctly?
If applicable, then verifies that these words are
used correctly. For example, the message
shortened to "BEACH USE EXIT 5".
Readability of the message and number of screens used.
12. Is the word "NEXT" used only as
Ensures that use of the word "next" is not
confusing when referring to an exit or tum.
Considers options such as naming the exit or tum
or replacing the word "next" with "first" or
Whether the exit or tum is visible once the message is read, whether
the road is an Interstate, and how well the exit or tum is marked with
static signs.
13. How should the VMS message be
If message is longer than 1 screen, then
determines how message should be divided.
Motorists' comprehension of individual screens if read by themselves.
14. Is the message acceptable?
Verifies that message wording is effective.
Abbreviations, local signing, order of message information, and
necessity of all screens.
15. How should the message be displayed?
Determines how to display the message with
respect to font and amount of time each screen is
Speed of traffic, increased legibility distances associated with single
stroke fonts and upper case letters, and avoidance of blank screens.
16. Does the VMS pass the drive-through
Drives past the VMS at least once to verify its
Visibility of VMS, readability of message, driver environment, and
the necessity of information conveyed.
17. When should the message be updated,
modified, or discontinued?
Determines when to change the message or
remove the VMS.
Message accuracy, timeliness of the message, current or expected
changes in traffic conditions, and motorists' reaction to the VMS.
Appendix A: Why should a VMS use no
more than two screens?
Steps through calculations showing how motorists
have a very limited amount of time to assimilate
VMS messages.
Limited flip-disk VMS legibility distance, concept of VMS being
outside of driver's field of vision when the VMS is very close to
driver, prevailing traffic speed, and amount of time required to read a
Appendix B: How should the VMS be
maintained when not in use?
Understands suggestions for cleaning the screen,
removing VMS from the road when not in use,
and charging the battery if necessary.
Need to reduce glare (by cleaning the Lexan screen), need to remove
VMS when there is no message to be displayed, and the need to
charge the battery for VMSs which have not been used recently.
Appendix C: What abbreviations are
Reads a list of recommended abbreviations (such
as "In" for "lane").
Commonly used words and abbreviations obtained from the literature.
FIGURE 3 (continued)
Miller et al.
The operator then realizes that not all screens are necessary: on the
two-lane road, "left lane closed" and "merge right" imply the s11me
message. The operator concludes Modules 14 and 15 with a onescreen message: "ACCIDENTj3 MijMERGE RT." In Module 16, a
drive past the VMS verifies its effectiveness: this drive-through can
reveal simple mistakes, such as the view of the VMS being blocked
by a pole or roadway curve. Finally, the last module ensures that the
operator either changes the message or removes the VMS as soon as
traffic conditions have returned to normal to ensure VMS accuracy.
In the previous scenario a variety of approaches could have been
considered that were not mentioned in the interests of time. For
example, had the major problem not been for motorists to merge but
instead for motorists to divert from the Interstate, the operator also
would have completed Module 7, which helps design a message for
diverting motorists to an alternate route. In that case the operator
also would have examined the need to use static signs for motorists
unfamiliar with the area. The emphasis of this example, however, is
to show how the modules can help the operator design an effective
VMS message for a particular situation.
are included within that module, such as placing the VMS before
major decision points in the most level area possible and where it is
accessible to maintenance vehicles. If any of the requirements are
not met, the operator is advised to consider either a different location for the VMS or an alternative form of communication, such as
a static sign, a ftagger, or highway advisory radio.
Figure 2 demonstrates the power of the manuals' guidelines for
assisting an operator with a complex decision: how to relay distance
information to the motorist. The operator is presented with each
piece of information in a binary decision format. By following this
decision tree an operator quickly learns whether a landmark, exit
number, or distance in miles should be used.
A few key concepts are emphasized throughout the manuals. For
example, the idea of giving the motorists a message that they can
read twice is repeated in several modules: first, the maximum number of screens is established on the basis of traffic conditions; second, the message is designed such that only essential components
are retained; third, the operator is asked to verify the message if it
is longer than one screen. In this manner, operators can be sure that
they have met basic VMS usage requirements without being prevented from using the VMS innovatively and effectively.
Two Sets of Manuals
A separate operator's manual was developed for permanent VMSs.
This manual is similar to the portable VMS manual, and in many
cases the modules are the same. However, the two manuals have
distinct differences.
The permanent VMS manual has only 16 modules because it
does not include a module describing where to place the VMS. In
addition, although portable and permanent VMSs both become
unreadable when the motorist drives close to them, the methods differ for computing the distance at which the VMS becomes unreadable. As shown in Equation 1, lateral distance is the key factor for
portable VMSs, whereas for permanent VMSs the problem is
vertical distance (3, 13). Thus, the appendixes that describe these
calculations are different for portable and permanent VMSs.
Furthermore, additional message purposes have been included for
permanent VMSs, as they have greater display capabilities. Finally,
example message components for the two manuals differ because of
their variance in display formats. For example, if one wants to convey that the two left lanes are closed, then a permanent VMS with
twelve characters per line can display a message such as "2 LFT
LANES I CLOSED" whereas a portable VMS with only eight characters per line might use a message such as "2 LEFT I LANES I
Consistency and Readability Built into Manuals
The single most important aspect of the manuals is that they are
users' manuals rather than mandates or a laundry list of messages.
They are essentially a training tool that encourages the creativity of
the operator and allows him or her to have the final decision in the
application of the VMS.
The manuals are simple and effective. They are highly readable
but substantial enough to help the operator ensure that the VMS is
used properly. Decision boxes placed in the same location on each
page step an operator through the manuals. The use of a "true-orfalse" procedure enables the operator to easily determine whether
each guideline has been met. For example, Figure 3 shows the third
module as determining the location of a VMS. Several requirements
Mechanisms for Feedback from VMS Operators
The authors realize that these manuals must evolve to accommodate
the needs of VMS operators if this work is to be useful. During
training courses based on the manuals, therefore, comments will be
solicited from VMS operators. One comment that has already been
received from VDOT traffic engineers is that these manuals are
more useful as a training tool and reference document than as a
pamphlet that should be carried to the field each time a VMS is used.
To reflect this change in emphasis, the guidelines are now formally
known as a 'user's manual. A second comment that has been
received is that the computational methods for determining the
maximum number of screens could be expressed as some simple
rules: one message screen is ideal, two are acceptable, and three
should be used only under extreme circumstances. Thus, although
the calculations for determining the amount of time a VMS is legible were used in the development of these manuals, it was not necessary for operators to redo those computations each time a VMS is
used in the field.
The traffic engineers also recommended the addition of a "drivethrough" module, explicitly stating that the operator should drive
past the VMS to assess its effectiveness, with attention to such
details as traffic or other obstructions that might hamper the view
of the VMS, the amount of time each message screen is displayed,
and a motorist's reaction if a VMS is not expected. Finally, a fourth
comment illustrates the importance of keeping the manuals consistent with VDOT practices in other areas. The engineers noted that
the word "next" had the potential to be confusing, as is the case with
static signs. Therefore an additional module was developed that followed practices outlined in previous research and the current
MUTCD guidelines (14). Additional comments of this nature will
be sought and used to improve the manuals.
It is critical to consider the needs of both operators and motorists to
maximize the effectiveness of VMSs. In developing VMS opera-
tors' manuals designed to help achieve this objective, essential
lessons were learned. Operators need a set of guidelines rather than
an extensive list of messages to fully use the capabilities of VMSs.
The manuals developed through this research are in a format easily
followed by an operator and they assist rather than replace the operator's decisions. Because they outline the thought process an operator should employ when using a VMS, the manuals may also serve
as a training tool.
Credibility must be maintained to maximize the effectiveness of
VMSs. VMSs should be used only to transmit essential information
about changed traffic conditions. Information limitations of VMSs,
confirmed by the literature and motorists, demonstrate that VMSs
should use no more than two screens, and use of only one screen is
preferred. Therefore VMSs should be used in conjunction with
other communications mediums if it is necessary to convey detailed
information to motorists. Finally, these manuals should be updated
as additional feedback from VMS operators and motorists becomes
This research report was supported by the Virginia Transportation
Research Council.
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The contents of this paper reflect the research of the authors but not necessarily the official views of VTRC.
Publication of this paper sponsored by Committee on Traffic Control
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