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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 19-10
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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. 19-11