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lighting that is mounted at the top of a sign structure pointing down may cast shadows during
the day that impair the legibility of the sign. Careful consideration must be given to placement
of sign lighting to balance daytime appearance and any concerns over upward facing light in an
airport environment due to stray light concerns.
Environmental conditions can affect sign visibility as well. Frost and dew forming on sign faces
can render retroreflective material ineffective23. These conditions are typically transient, and the
material recovers without intervention. In areas of high humidity and frequent frost or dew con-
ditions, sign lighting may be required to overcome this material deficiency.
3.5.3.3 Internal Illumination
Internally illuminated signs can be designed to provide sign recognition and legibility dis-
tances comparable to those that are externally illuminated. Proper materials and design must be
used for the specific viewing angles present for a specific sign location. Candidate sign materials
should be viewed in daylight and dusk conditions to ensure that there is adequate contrast when
the sign is not lit. Internally illuminated signs might also help address concerns with stray light
expressed with external illumination.
3.6 Changeable Message Signs
A Changeable Message Sign (CMS) is a lighted matrix sign that is capable of displaying messages
that may be difficult to communicate with standard static signing. These signs are also referred to
as Dynamic Message Signs (DMS) or Variable Message Signs (VMS), and these terms are used
interchangeably throughout this section. These signs can be mounted in a permanent manner
beside or over a roadway, or they may be placed on portable trailers. Small CMS signs may also
be placed within a larger static sign, such as when available parking spaces are displayed. Exam-
ples of CMS signs are shown in Figures 3.9 and 3.10.
3.6.1 Appropriate Use of Changeable Message Signs
The primary function of a CMS is to alert and inform motorists of changing or temporary
conditions along their travel path. Changeable signs on major roads should be used only to con-
vey messages that change on an hourly, daily, or weekly basis, such as:
· Traffic conditions and roadway delays,
· Construction or maintenance lane closures or speed reductions, and
· Homeland Security threat level.
Figure 3.9. Example of CMS parking information
(photos courtesy of Daktronics, Inc.).
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48 Wayfinding and Signing Guidelines for Airport Terminals and Landside
Photo credit: Chris Cunningham
Figure 3.10. Tampa International Airport economy
parking garage.
In other locations CMS may be used for:
· Parking availability and
· Parking fee schedules.
3.6.1.1 CMS vs. Static Sign
Changeable signs should be used where they are clearly beneficial to accommodate a changing
state of operations and to provide convenient and timely information to the passengers and patrons.
It is FHWA policy that the appropriate use of a CMS and other types of real-time displays on
the normal roadway system should be limited to managing travel, controlling and diverting traf-
fic, identifying current and anticipated roadway conditions, or regulating access to specific lanes
or the entire roadway24. Airports represent a different environment that has not been investi-
gated by FHWA, but many of the same principles and guidance exist in those applications as well.
CMS are one of the primary links between a transportation agency and those using the trans-
portation facility. The design, display, and management of messages on CMS introduce many
challenges for transportation agencies, airports included. Some of these challenges are addressed
in The Dynamic Message Sign Message Design and Display Manual25 and The Portable Changeable
Message Sign Handbook26. These documents provide comprehensive information on the use of
changeable message signs in the typical roadway environment and will be helpful in assessing the
use of these signs for an airport.
In any case, the CMS should not replace permanent, static signing required in the MUTCD.
For construction activities, portable, trailer mounted CMS may be used.
A CMS can be an effective permanent and/or temporary traffic control device when used
appropriately and coordinated with static signing systems. However, this effect can be dimin-
ished if this device is overused or improperly used. Each airport operator should establish or
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endorse a written policy regarding the use of CMS and the authority to display messages should
be limited. Many state transportation agencies have published CMS guidelines that may be help-
ful references.
3.6.2 CMS Technology
Each sign consists of a sign face, the sign housing, and a controller. The signs can be permanent
or portable. Permanent signs can vary in size, but normally can display three lines of text with 12 to
18 characters per line. Portable CMS typically have a modular display consisting of three lines of
eight characters each. New full matrix sign displays provide the most flexibility, including the abil-
ity to vary the height of the characters, display simple graphics, and use proportionally spaced fonts.
The most common types of lights used in CMS include: light-emitting diode (LED), fiber
optic, incandescent bulb, and reflective disk. A description of these sign types is included in the
Texas Transportation Institute (TTI)'s Guidelines on the Use and Operation of Changeable
Message Signs27.
Control of the sign display can be accomplished via direct connection with a laptop or any
number of other methods via a remote computer communicating through an Ethernet modem
connection over copper cables, fiber optic cable, radio frequency (RF) links, or cellular transmis-
sion. In any case, the use of National Transportation Communications for ITS Protocol (NTCIP)
is required to comply with ITS industry communication standards.
3.6.3 Message Design and Layout
When CMS are used on airport roadways, their messages must also be carefully constructed to
avoid information overload. A CMS must communicate a meaningful message that can be read and
understood by motorists within a very short time period. A motorist's ability to read and under-
stand a message depends upon the message load, which refers to the units of information in the total
message. CMS messages must also "fit" within and work with the static sign system layout.
Lighting conditions vary throughout the day, based on the position of the sun and driver rel-
ative to the message sign, and can impact the driver's ability to read a CMS. Common lighting
conditions include: mid-day (sun is overhead), washout (sun is behind driver), backlight (sun is
behind sign), and nighttime (no sun). Attention should be given to ensure that CMS messages
are readable under prevailing lighting conditions at the location at which they are intended to be
used. MUTCD requires that CMS automatically adjust their brightness levels to account for vary-
ing light conditions.
Other message design features include message content, length, and format. Message content
refers to specific information displayed on a CMS. Key elements of message content include
information about the event or problem ahead and what the motorist should do about it. Mes-
sages are most often displayed in one or two multi-unit message phases. Although the use of
three phases is possible, it is not recommended. For roadway applications, FHWA recommends
the following guidelines for CMS message phases:
· Single Phase Display
Line 1--Describe the problem.
Line 2--Identify location or distance ahead.
Line 3--Provide motorist instruction.
· Two-Phase Display
Phase 1--Describe the problem.
Phase 2--Provide motorist instruction.
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50 Wayfinding and Signing Guidelines for Airport Terminals and Landside
Message length refers to either the number of words or the number of characters and spaces in
a CMS message. With obvious limitations on CMS line capacity, it is usually necessary to deter-
mine if the message will fit, then decide if abbreviations can be used or redundant words can be
eliminated. Acceptable phrases and abbreviations should be well documented in the airport
operator's policy regarding the use of CMS. Table 1A-2 of the 2009 MUTCD gives acceptable
abbreviations for use on changeable message signs.
Message format refers to the order and arrangement of the units of information on a CMS.
Motorist comprehension and decision-making is enhanced when the proper information is
presented in the expected order. Poor formatting can result in driver confusion and increased
reading times.
More information regarding message content, length, and format can be found in The Dynamic
Message Sign Message Design and Display Manual, The Portable Changeable Message Sign Handbook,
and the 2009 MUTCD.
Each phase of a CMS should be displayed long enough to allow the motorist to read it; how-
ever, unnecessarily long display times should be avoided. A phase refers to a message segment
that is individually displayed, for example, the first phase may be "Road Work Ahead" and the
second phase "Speed Limit 30 mph."
The following display times are recommended in Section 2L.04 of the 2009 MUTCD:
Guidance:
· The minimum time that an individual phase is displayed should be based on 1 second per word or 2
seconds per unit of information, whichever produces a lesser value. The display time for a phase
should never be less than 2 seconds.
· The maximum cycle time of a two-phase message should be 8 seconds.
· The duration between the display of two phases should not exceed 0.3 seconds.
· Compatible units of information should be displayed on the same message phase.
Two phases should only be used when absolutely necessary and when both phases contain
no more than one unit of information (i.e., the answer to a What? Where? When? question
constitutes one unit of information.) Since drivers may first be able to read sign just at the
moment the phase changes, the sign letter height, in combination with operating speed, must
be designed so that the driver has the opportunity to read three phases per message for a two-
phase message. Drivers require at least two seconds of display time per unit of information. A
16-inch letter height sign can be read at a distance of 480 ft (legibility distance is 30 feet per
inch of letter height). To display a message with three phases each requiring two seconds, for
a total of six seconds, the maximum operating speed would be 55 mph (480/6 = 80 ft/sec, or
55 mph).
3.6.4 CMS Display Elements
3.6.4.1 Letter Size
Legibility experiments of CMS character matrix technology had indicated that 18-inch char-
acters can be read far enough away by most drivers in optimum daytime viewing conditions to
provide 8 seconds of available viewing time while traveling at 70 mph. Another legibility study
found that 12-inch characters were sufficient at 45 mph during optimum conditions28. However,
optimum conditions often do not exist due to weather and/or other factors.
The MUTCD provides further guidance regarding letter height in Section 2L.04:
Word messages on changeable message signs should be composed of all upper-case letters. The mini-
mum letter height should be 18-inches (450 mm) for changeable message signs on roadways with speed
limit of 45 mph (70 km/h) or higher.
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Based on this information, a minimum letter height of 12 inches is recom- Operating Recommended
mended for roadways with operating speeds less than 45 mph, and 18 inches for Speed (mph) Letter Height
(inches)
roadways with operating speeds of 45 mph or more. Recommended letter heights Less than 45 12
are shown in Figure 3.11. mph
45 mph or 18
3.6.4.2 Font more
New CMS technologies allow denser pixel placement and a wider variety of fonts Figure 3.11. Recommended CMS
and letter forms than in the past. The MUTCD does not specify a font to be used on letter heights.
CMS. It does provide guidance statements that encourage minimum visibility and
legibility distances in Section 2L.03 Paragraph 4. Additional sections (2L.04) provide guidance on
letter stroke width, height: width ratio, character spacing, and use of upper and lower case letters.
3.6.4.3 Color
Typical roadway CMS signs generally contain white, yellow, amber, or fluorescent yellow-
green legends on a black background. Full matrix LED sign displays are also capable of display-
ing full color text and graphics, but use of some of these colors may actually increase driver
recognition and comprehension times. Although these colors may have greater target value than
other colors, the novelty of these signs may prevent their recognition as traffic control devices.
The sign structural support and housing materials should be selected with care so as not to
distract from the message being displayed. The housing should also be neutral enough that the
CMS is recognized by drivers as a traffic control device and not an advertisement.
In support of this concept, the MUTCD contains the following language in Section 2L.02:
Standard: When a changeable message sign is used to display a safety, transportation-related, emergency
homeland security, or AMBER alert message, the display format shall not be of a type that could be con-
sidered similar to advertising displays.
Research shows that white or amber is preferred for CMS letter colors. The use of red is not
recommended.
In addition, Section 2L.04 of the MUTCD discourages the use of CMS in a manner that may
be distracting to drivers:
Standard: Changeable message signs shall not include advertising, animation, rapid flashing, dissolving,
exploding, scrolling, or other dynamic elements.
3.6.5 Passenger Advisory CMS
With security taking a high priority in airport activities, there is a need to communicate ele-
vated risks to travelers. The Homeland Security Advisory System uses a color-coded system to
place citizens and public officials on notice about the likelihood of a terrorist attack. The more
specific and imminent the threat, the higher the threat level is set. The threat level changes peri-
odically, so changeable message signs can offer an avenue for communicating these changes in
the airport environment.
If public agencies decide to display emergency or security alert messages on a CMS, FHWA
has determined that this application is acceptable if public agencies have developed policies and
procedures that govern the messages that are displayed on CMS and their operation. The pub-
lic agency policy and procedures relating to displaying emergency or security alert messages on
CMS must address the following issues:
An FHWA memorandum dated March 21, 2003, addresses Use of Changeable Message Sign
(CMS) for Emergency Security Messages64:
· The criteria under which CMS will be used for emergency or security alert messages, includ-
ing the necessary coordination with public safety or security agencies. Formal policies among
