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OCR for page 45
HFG CHANGEABLE MESSAGE SIGNS Version 1.0
DISPLAYING MESSAGES WITH DYNAMIC CHARACTERISTICS
Introduction
Dynamic characteristics refer to message properties that specify character movement. These characteristics include
the time to display each message phase, blanking between phases of a multi-phase message, flashing one or more
lines of a message, alternating lines in multi-phase messages, and looming (making text or symbols increase in size
over time). Improper use of dynamic message characteristics can lead to increased reading times and reduced
message comprehension.
Design Guidelines
Topic Definition Guideline Rationale/Source
Phase Display The amount of time to display each Use whichever is longest: Research and field
Time phase of a two-phase message 2 s per information unit or experience (1)
1 s per four- to eight-character
word (excluding prepositions)
Blank Time The amount of time that a CMS is left Insert a 300 ms blank screen Increased word and
between Phases completely blank between message between message phases 1 and 2. number comprehension (3)
phases
Flashing One-phase messages that flash the entire Do not use. Disagreement in research
Messages message results (4, 5)
One-phase messages that contain one Do not use. Increased reading time and
flashing or blinking line reduced comprehension
(4, 5)
Alternating- Multiple-phase messages in which only Do not use. Increased reading time
Line Messages a subset of the lines change between (4, 5)
phases
Looming Increasing text or symbol size over time Do not use. No positive effect (3)
Based Primarily on Based Equally on Expert Judgment Based Primarily on
Expert Judgment and Empirical Data Empirical Data
BLANK TIME BETWEEN CYCLES (FROM DUDEK (1))
TYPE OF CMS EXAMPLE BLANK TIME BETWEEN CYCLES
One-word or one-line sign 0.25 s blank screen +
with three or more phases 0.50 s screen with 3 asterisks +
0.25 s blank screen
OR
0.25 s or less between phases +
1.00 s between cycles
EQUATION: HOW MUCH TIME SHOULD BE USED TO DISPLAY EACH PHASE?
1. Find the time that is available for the entire message Legibility Distance (ft)
T = total time available to read the message T(s) =
Traveling Speed (ft/s)
2. Find the time that is needed for each phase
Time for phase 1 (t1) = 2x
x = number of information units in phase 1
y = number of information units in phase 2 Time for phase 2 (t2) = 2y
3. Make sure that the time required is less than or equal to the
time available T B + t 1 + t2
B = blanking time between phases
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OCR for page 46
HFG CHANGEABLE MESSAGE SIGNS Version 1.0
Discussion
Only a limited amount of research has been conducted on the dynamic properties of message signs (2). In addition,
most of the studies have been conducted in laboratory or simulator settings rather than on the road.
Phase display time: The amount of time that a single phase should be displayed is determined by the amount of
content in that phase. Dudek (1) summarizes that either 1 s is needed per four- to eight-character word excluding
prepositions or 2 s is needed per information unit, whichever is longest. The total time available to divide between
the phases is reduced by the blank time between the phases, discussed below.
Blank time between phases: Greenhouse (3) found that inserting a 300 ms blank screen between phase 1 and phase 2
of a portable message sign improves comprehensibility. This improvement is possibly because a refractory period
helps information processing between screens. Although this conclusion applies directly to portable message signs,
it may be true for permanent message signs as well. Note that the blank screen was only tested between phase 1 and
phase 2, not between phase 2 and phase 1 when the message cycled. It is unknown if providing a blanking time
between phase 2 and phase 1 would provide a further benefit. It is reasonably conceivable that drivers who see a
blank between phases 1 and 2, but not between phases 2 and 1, would reverse the order of the phases and possibly
have trouble understanding the message. Dudek (1) recommends that blank time and/or asterisks be displayed
between cycles of a message that contains three or more phases (on one-word or one-line signs). Because these signs
are more limited in the amount of information that they can display at one time, the phases may not make sense
independently and drivers who read later phases before phase 1 may not understand the message. Thus, giving an
indication of where the message is in the cycle gives drivers an idea of their location in the cycle.
Flashing phase: There are many ways in which all or portions of messages can be flashed in an attempt to draw
driver attention. One method is to flash the entire display for a one-phase message. Research (4, 5) in laboratory and
simulator settings disagreed with regard to the effects on comprehension and reading time. In the laboratory,
comprehension was not affected, but reading times were significantly longer when the message was flashing. In the
simulator, comprehension was negatively affected for unfamiliar drivers, but reading times were not affected. Full-
phase flashing messages are not recommended because of this disagreement in research results.
Flashing line: Another flashing method is to flash one line of a message. Research in laboratory and simulator
settings (4, 5) showed that comprehension levels and reading times were both negatively affected by this method.
Thus, flashing one line is not recommended.
Alternating line: In alternating-line messages, a portion of the message is held constant between the two phases
(usually the first two lines) while the other portion is alternated between two pieces of information (usually the third
line). Research (4, 5) on this method showed that although comprehension was not affected, reading times greatly
increased.
Looming: In a study by Greenhouse (3), looming was shown to negatively affect some driver demographics more
than others. However, it did not help any group of drivers comprehend messages. It also seemed to function as an
additional driver distraction and a negative effect on intelligibility.
Design Issues
None.
Cross References
Composing a Message to Maximize Comprehension, 19-8
Key References
1. Dudek, C.L. (1992). Guidelines on the Use and Operation of Changeable Message Signs. (FHWA-TX-92-1232-9). College Station: Texas
Transportation Institute.
2. Dudek, C.L. (2004). Changeable Message Sign Operation and Messaging Handbook. (FHWA-OP-03-070). College Station: Texas
Transportation Institute.
3. Greenhouse, D. (2007). Optimizing Comprehension of Changeable Message Signs (CMS). (UCB-ITS-PRR-2007-24). Berkeley: University
of California Partners for Advanced Transit and Highways (PATH).
4. Dudek, C.L., and Ullman, G.L. (2002). Flashing messages, flashing lines, and alternating one line on changeable message signs.
Transportation Research Record, 1803, 94-101
5. Dudek, C.L., Schrock, S.D., and Ullman, G.L. (2005). Impacts of Using Dynamic Features to Display Messages on Changeable Message
Signs. (FHWA-HOP-05-069). Washington, DC: FHWA.
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