National Cooperative Highway Research Program Synthesizes Research on
Changeable Message Signs (CMSs)
Professionals in the fields of highway design, maintenance, and safety have a
substantial amount of information and research available to help them in their work;
however, this information is scattered and not easily accessed. The American
Association of State Highway and Transportation Officials (AASHTO), in association
with the National Cooperative Highway Research Program (NCHRP), has authorized a
continuing project to consolidate, synthesize, and make available this storehouse of
information. The NCHRP's Synthesis 237, entitled "Changeable Message Signs," is
one example of this effort.
As the name suggests, "changeable message signs" change. They are signs that give
the motorist real-time information about roadway congestion, work zone detours, closed
bridges, or ice on curves. They say different things at different times on different days.
Changeable message signs (CMSs), also known as variable message signs or motorist
information displays, are widely used in the United States and Canada.
Synthesis 237 provided information on the various types of CMSs used, including their
characteristics, technology, character (letters and numbers) types and size, and degree
of visibility. Since each type of CMS is different, each offers different advantages,
depending on the needs of the agency using the sign. Synthesis 237 highlighted those
differences and advantages, so agencies could make informed decisions about current
or anticipated use of CMSs.
Researchers designed a questionnaire survey and mailed it to each state department
of transportation representative on AASHTO's Subcommittee on Traffic Engineering.
Surveys were also mailed to one turnpike authority (New Jersey) and two Canadian
provinces (Ontario and Saskatchewan). The survey had two parts: one for
permanently mounted CMSs and the other for transportable CMSs.
Forty-two agencies (39 states, the turnpike authority, and the two provinces) responded
to the survey; all these agencies responded to part one, and 35 responded to part two.
Only responses to part one of the survey were included in the synthesis. Responses to
part two were not as complete as those for part one, and the information on
transportable CMSs did not add to the information available on permanent CMSs. The
survey was conducted in 1992, updated in 1995, and reflected the state-of-practice as
of December 1995.
There are essentially three types of CMSs: light-reflecting, light-emitting, and hybrid.
They "form characters and symbols in a matrix format by showing appropriate patterns
of the matrix elements." The three types of CMSs use one of three matrix formats: the
modular character matrix, continuous line matrix, and full matrix--as illustrated in
Figures 1, 2, and 3, respectively. The full matrix format is the most flexible but is also
the most costly.
Reflective disks and rotating drums are examples of light-reflecting CMSs. They reflect
light from an external source, such as the sun or vehicle headlights. Unlike the rotating
drum CMS, which has a limited number of messages for display, the reflective disk
CMS can display an infinite number of messages. Reflective disk CMSs are divided
into three main types--circular disks, rectangular disks, and dimensional square disks.
Whatever the type, all light-reflecting CMSs require power only when a message is
changed (although they also use power for external and internal lighting needs and
environmental controls such as fans and heaters).
Like light-reflecting CMSs, light-emitting CMSs require power for fans and heaters.
Unlike light-reflecting CMSs, they also require continuous power when a message is
displayed. Light-emitting CMSs may use any of the three matrix formats described
above. The most common types of light-emitting CMSs are the bulb (incandescent)
matrix, the fiberoptic matrix, and the light-emitting diode (LED) matrix. The bulb matrix
(also known as lamp matrix) is one of the oldest types of light-emitting CMSs used for
highways. Recent technological advances account for the increased popularity of
fiberoptic and LED signs.
Fiberoptic CMSs are either fixed grid, with a fixed number of available messages, or
matrix with shutters. The matrix with shutters has more flexibility, because it can
display a significant number of user-designed messages. Except for lane control
signals, the fixed grid fiberoptic CMS is not routinely used for traffic management on
U.S. freeways. As such, references in this report to fiberoptic CMSs will be to the
Hybrid CMSs combine two types of CMS technology for a sign that shares the qualities
of both. Recently the technologies of fiberoptics and LED have been integrated with
circular reflective disk matrix technologies. The hybrid operates on the established
principles of reflective disk sign technology, supplemented with fiberoptics or LED.
PERMANENTLY MOUNTED CMSs
In the survey, 27 states, the New Jersey turnpike authority, and Ontario indicated they
use permanently mounted CMSs in the following nine ways:
- General Traffic Information/Warning
- Incident/Traffic Management
- Diversion Information
- Construction/Maintenance Support
- High Occupancy Vehicle/Contraflow Lane Information
- Reversible Lane Control
- Special Event Traffic Control
- Fog Warnings
- Warnings of Adverse Weather/Road Conditions
There is some overlap among the first three applications. Moreover, the responding
agencies use these nine applications in various ways, from a single purpose such as
overhead clearance to a wide variety of purposes such as incident and traffic
management and diversion.
The survey revealed certain trends in the purchase of CMS types. For example, most
purchases from the mid 1970s to the early 1980s were light-reflecting technologies,
namely the circular reflective disk and rotating drum signs. Since 1989, the trend has
been toward purchase of light-emitting models--fiberoptic, LED, fiberoptic-enhanced
reflective disk hybrid, and LED-enhanced reflective disk hybrid CMSs. When agencies
began to focus on energy conservation in the 1980s, the circular reflective disk CMS
was quite popular. To improve legibility, however, some agencies are now retrofitting
these older reflective disk signs with fiberoptic-enhanced reflective disk modules. Only
Connecticut reported using dimensional square reflective disk signs. Agencies in
Minnesota and other northern states routinely use the rotating drum sign, despite its
limited number of messages. California has adopted a full-matrix bulb CMS as the
state standard, although this older type of CMS has generally declined in popularity.
Most recent CMS purchases are of line matrix signs with three lines of text, though
agencies in California and Connecticut have purchased full matrix signs capable of
displaying a variety of lines and character heights. The majority of recent rotating drum
purchases have three lines of characters, though some with one or two lines have been
purchased for a particular use--such as for weigh station signs.
Essentially all highway CMSs display messages in all capital letters, while the number
of characters per line on recent purchases varies from 10 to 24, depending on need.
An agency should consider a number of recommendations before purchasing a CMS;
such consideration helps insure that the purchased CMS will meet the needs and
requirements of the agency. Establishing in advance the sign's objectives and
messages should help achieve adequate character space and legibility. Long
messages may require abbreviations or two-phase messages. While some agencies
have successfully used two-phase messages, respondents in Ontario expressed
concern that two-phase messages reduce message exposure time to drivers by up to
50 percent. Consequently, Ontario plans to require a minimum of two lines of 21
characters on new CMSs to guarantee that entire messages can be displayed at once.
There is also concern about the maximum length of message drivers can adequately
and safely read at prevailing speeds. Researchers recommended that messages be
exposed to drivers at a rate not to exceed one word of information per second. At a
prevailing speed of 55 mph, the message should not exceed eight words (excluding
prepositions and assuming a typical legibility distance of 650 ft.). With lengthy
messages or adverse and fluctuating environmental conditions, drivers may have to
reduce their speeds to read the message.
Studies have also shown that CMSs used on U.S. freeways should have character
heights of at least 457 mm (18 inches) to handle message requirements for most
applications and audiences. These signs need excellent visibility to compete with the
commercial electronic advertising signs typical of urban and suburban environments.
Letter heights between 254 and 457 mm (10 and 18 inches) are recommended for
roads other than freeways.
How well a CMS can be seen depends on two factors--the visual capabilities of drivers
and the photometric qualities of the sign. Conspicuity or target value is defined as "the
ease with which the sign is first noticed and can be detected in the driving
environment"; legibility is "the ease with which the message can be read."
How effective a CMS is depends largely on how much time a driver has to read it; this
available time is determined by travel speed, the distance from the sign when it is first
noticed, and the legibility distance of the sign. Drivers must recognize a CMS and its
message before they can respond to it, and a sign's brightness is a significant factor in
its overall visibility. This brightness, known as luminance, is defined as the amount of
light emitted or reflected by a surface. Luminance is directly affected by time of day
and weather. On a bright, sunny day, the luminance must be significantly brighter for
contrast. This potential contrast problem is acute when the sun's rays are directly
behind or in front of the sign.
Only a few experimentally controlled studies in the U.S. have been conducted to
provide data on the legibility of light-reflecting and light-emitting matrix CMSs. Those
studies found that the legibility distances for bulb matrix CMSs are about 15 percent
longer than reflective disk CMSs with a single line. Regarding visibility, subjective
studies found the bulb matrix superior to the disk matrix at night, in overcast skies, at
dusk, and when the sun is to the rear of the sign.
Table 1 shows the results of a 1991 study measuring the legibility distances of
clustered LED (452-mm (17.8-inch) characters), fiberoptic (409-mm (16.1-inch)
characters), and reflective disk matrix (457-mm (18-inch) characters) CMSs under four
lighting conditions. While the researchers did not control the contrast ratios of the
signs to produce identical conditions, the study nonetheless shed light on legibility
distances. LED lamp and sign technology is more advanced now than in 1991, so
further studies are warranted to evaluate these improved LED CMSs.
Average Legibility Distances for LED and Fiberoptic Signs Under Different
Lighting Conditions (9,10)
||Legibility Distance, m (ft)
Twenty (71 percent) of the responding agencies said they operate their CMS systems
with messages chosen from a computer library, and supervisors create special
messages when needed. Ten agencies (36 percent) reported that trained operators
create messages during emergencies when the available library messages do not fulfill
the need. Nineteen agencies (68 percent) display messages manually using CMS
operators; ten agencies (36 percent) activate the signs primarily with operators, but do
use some form of automatic intervention as well. Eight agencies (29 percent) operate
signs automatically with some operator intervention; and some of the CMS systems of
seven agencies (25 percent) are fully automatic.
Splitting and Sequencing Messages
Some situations require messages that are longer than a sign is capable of displaying
at one time. These situations require sequenced messages (also known as alternating
or multi-phased messages), which means the message is spilt into two or more parts
that are sequenced on the CMS.
EXPERIENCES WITH CMS TECHNOLOGY
Agencies responding to the survey were asked to highlight their experiences with
CMSs, including the signs' best and worst features and what the agency might do
differently in future experiences with CMSs.
Reflective Disk (Circular)
Agencies responded with a number of positive attributes for the popular reflective disk
CMS, including low power usage, a relatively low purchase price, and overall reliability.
This technology was given low marks for reduced legibility under certain conditions,
especially when the sign faces east or west. The problem is compounded by fading of
the reflective coating on the flip disks and yellowing and deterioration of the sign's
plexiglass face. To improve the legibility of the sign, some agencies have tried
retrofitting existing reflective disks with fiberoptic-enhanced reflective disks.
Reflective Disk (Three-Dimensional Cube)
Connecticut was the only respondent using this CMS. The sign's main advantage is
the ability to use the full matrix for displaying a variety of character fonts, sizes, and
graphics. On the down side, the cubes are prone to sticking, but Connecticut agencies
have adopted a practice of daily tests to keep the cubes mobile.
Ten of the agencies had recently purchased rotating drum CMSs--a technology with
ongoing popularity, particularly in northern states. The rotating drum was reported to
be relatively inexpensive to purchase and maintain, reliable, easy to service, and
legible. Its major drawback is the inability to display as many messages as matrix-type
Alabama and California recently purchased bulb matrix CMSs. They believe it offers
the best legibility under all environmental conditions; and, unlike some light-emitting
CMSs, the bulb matrix CMS can be read from many angles and locations. However,
the bulb matrix CMS is expensive to operate and maintain.
Eight agencies were recent purchasers of this technology, which offers both good
conspicuity (target value) and legibility. It is not as readily visible from a variety of
locations; and, therefore, crews must take extra care in the placement and positioning
of the sign.
Light-Emitting Diodes (LED)
Improvements in this sign's luminance have made it more popular in recent years.
Technology now allows the use of amber LEDs, whereas earlier models had to
combine red and green LEDs to simulate amber.
Colorado is using 33 LED signs with good results but has experienced the color
problems inherent in mixing red and green to produce amber. Maryland reported an
unusual and problematic connection between certain types of non-prescription
sunglasses and LED signs. In construction zones, the sunglasses seemed to block
messages on CMSs with amber LEDs, which essentially rendered the messages
invisible to drivers wearing these sunglasses. As a result, the Federal Highway
Administration (FHWA) issued a policy statement that defines the spectrum of amber
within which the sunglasses create visibility problems.
Fiberoptic-Enhanced Reflective Disk
Six agencies have purchased these signs and reported good conspicuity and legibility
in all directions and good reliability. As a result, four agencies were replacing disk
modules with fiberoptic-enhanced reflective disk modules on many of their current
circular disk CMSs.
LED-Enhanced Reflective Disk
Washington and Wyoming have used this technology, and Washington reported high
visibility in all directions. The signs have not been used long enough to offer any other
FUTURE CMS TECHNOLOGY PURCHASES
The agencies were asked which CMSs they would consider for future purchase and
which they would likely avoid. While some agencies indicated they would consider
light-reflecting CMSs, the newer light-emitting CMSs seemed the sign of choice. Nearly
all (22 of 26) agencies said they would not consider purchasing the bulb matrix CMSs,
but almost all said they would consider other light-emitting signs.
The agencies were asked if they had developed CMS standards in a number of
categories, and most responded they do not have statewide standards governing the
use of CMSs. National standards for CMSs still have a long way to go, but there have
been considerable improvements in recent years. The lack of standards is due largely
to the relative newness of CMS use, despite their introduction in the early 1960s.
Agencies were also asked about their experiences with contrast ratio and legibility.
Contrast ratio describes how legible a sign is during daylight conditions. Despite the
significance of contrast ratio, there is no formal mechanism in the U.S. for measuring
contrast ratio when a sign purchase is considered.
Some agencies showed an ongoing dissatisfaction with the conspicuity and legibility of
certain CMSs; however, they had not established criteria to address these concerns.
Report authors suggested that if agencies hope to improve the visual performance of
their CMSs, they must establish objective and carefully conceived criteria.
When asked what they would do differently in the future regarding standards,
specifications, and maintenance agreements, about a third (30 percent) of the agencies
said they would improve their standards and specifications. The fact that 21 percent of
the agencies said they would consider other CMSs for future purchase highlights a
need for more thorough evaluation of a CMS before purchase.
In Ontario, Delcan Corporation conducted a study on CMS failures; however, the lack
of comprehensive data did not permit definitive conclusions. Analysis of down-times for
the CMS indicated that 45 percent were the result of communication failures, 30
percent the result of bulb-outage failures, 13 percent the result of power failures, and
12 percent the result of miscellaneous causes.
Maintainability is the ease with which a CMS can be accessed for maintenance and, at
the same time, ensure workers' safety and minimum disruptions to the flow of traffic.
Several factors relate to maintainability, including a sign's location and access to its
controls and components.
Agencies expressed concern about pressure to accept low bids from manufacturers,
feeling it might mean a loss in quality and maintainability. In addition, if a manufacturer
goes out of business, CMS owners are left without access to parts or repair. Agencies
also voiced significant concern about incorporating a new CMS into an existing system.
The requirement to accept the lowest bid could result in hardware incompatibility
between the new and existing systems. Most agencies that responded to the survey
felt steps need to be taken to correct these problems, and they generally agreed that
carefully conceived and precisely written specifications for manufacturers would
provide a measure of protection. Specifications should also be tailored to the specific
agency, since each agency has unique requirements for its CMSs.
Ten of 24 (42 percent) agencies indicated they were not familiar with research and
publications on CMSs, which suggests that the agency personnel responsible for CMSs
were not receiving and/or using information that would help them make informed
decisions about the CMSs they have or expect to purchase.
Responses to the survey confirmed that CMS technology has improved in recent years,
but results also showed that certain aspects of CMSs remain in need of research and
action to improve the overall quality of CMSs and facilitate their use. The major
conclusions drawn from the study are:
- One method of insuring the purchase of quality CMSs would be to write very
specific, detailed specifications for the unique requirements of the purchasing
- The survey suggested the overall need for a national standard communications
protocol and a national testing facility to provide consistency and uniformity in
the technology and use of CMSs.
- Field tests are needed to provide objective data on the characteristics and
performance of CMSs. Currently CMS purchasers rely primarily on the more
subjective information provided by CMS manufacturers. Additional field tests are
also needed to gauge driver response to CMSs. Few studies have measured
driver response to CMSs in real situations, and the studies that do exist are 20
years old. Field tests might also offer objective reasons why drivers sometimes
slow their speeds at certain CMS locations when messages are displayed.
- Access to the latest research and information on CMSs would greatly benefit the
people directly responsible for the purchase and maintenance of CMSs. In
addition, an improved and quicker exchange of information among transportation
agencies about alternative CMSs, and particularly those with new technology,
would benefit all agencies.
- Research highlights the need for a minimum character height of 457 mm (18
inches) for matrix CMSs on freeways.
- Research would be helpful to determine if messages should be displayed on
CMSs during nonincident situations and also if non-traffic-related messages
should be displayed on CMSs.
- Studies are warranted concerning the overall operation and effectiveness of
- Transportation agencies that provide careful documentation about the use of
their CMSs will figure significantly in the design process of future CMSs.
Copyright © 1997 by TranSafety, Inc.
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