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HFG CHANGEABLE MESSAGE SIGNS Version 1.0 DETERMINING APPROPRIATE MESSAGE LENGTH Introduction Determining the appropriate message length for a CMS refers to choosing a message length that drivers have the time to comprehend as they pass the sign. Controlling message length is extremely important because there is a limited amount of time to present information to drivers. Message length is described not only by the absolute length in the number of words, but also by the number of information units included in these words. Information units are a measure of the message load, or total amount of information in the message. If there are too many words or information units in a message, it may need to be split into two phases. Dudek (1) provides additional guidance for reducing message length and splitting long messages. Design Guidelines Message Guidelines Example (from Dudek (1 )) Property Information Use no more than: Units: 2 information units per line A measure of the Answer amount of 3 information units per phase Question (see below) (1 information unit) information presented in terms 4 information units per What is the problem? MAJOR ACCIDENT of facts used to message read at speeds of Where is the problem? AT US-23 make a decision; a 35 mi/h or more single information 5 information units per Who is the message for? NEW YORK unit consists of message read at speeds less What should they do? USE I-280 EAST 1 to 4 words than 35 mi/h Length: Use no more than: Number of words Eight words per message for or characters in a drivers at high speeds message, (based on the required reading excluding time of 1 s per four- to eight- prepositions character word, excluding prepositions, or 2 s per Acceptable message length because the preposition "to" does information unit, whichever is not count. longest) Phases: Two phases maxi mum per Poorly designed message: Similar to a page message of a book, a phase Each phase must be able to be is the text that is understood alone displayed at a single point in time One line should not contain parts of 2 information units but may contain 2 whole Improved message: information units When dividing messages between two phases, compatible information units should be kept on the same phase Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 19-6
HFG CHANGEABLE MESSAGE SIGNS Version 1.0 Discussion Information units: The recommendations for the number of information units that are appropriate for display are based on research and operational experience (1). Dudek (2) summarizes that 1 s is needed per four- to eight-character word excluding prepositions or 2 s per information unit, whichever is longer. Using this assumption, the required viewing distance for different numbers of information units, for drivers traveling at different speeds, are included below. REQUIRED VIEWING DISTANCE PER NUMBER OF INFORMATION UNITS AT VARYING SPEEDS (FROM DUDEK (1 )) However, the MUTCD (3) states that the minimum phase display time should be based upon 1 s per word or 2 s per information unit, whichever is shorter. This direct contradiction of Dudek (2) causes practical consequences for drivers. If the longer time is used (2) and the message includes many short words that do not all need to be remembered, the phase display time could be unnecessarily inflated. If the shorter time is used (3) and the message includes information units composed of multiple important words, drivers may not have time to read all of them. If longer messages need to be provided, they should be shown in conjunction with other information media (See When to Use Changeable Message Signs, 19-2). Length: Dudek (1) states that the appropriate absolute message length is affected by (1) the amount of time that the driver is in the legibility zone of the CMS, considering travelling speed and environmental conditions; (2) the driver workload including all driver activities such as reading signs, lane positioning, etc.; and (3) message familiarity because drivers take more time to read unfamiliar content or unusual messages The eight-word maximum for high speeds is based on the legibility distance, or the distance at which the words on the sign become legible, as well as the speed that the driver is travelling. This recommendation assumes drivers are traveling at 55 mi/h and the legibility distance of the sign is 650 ft (which is the approximate legibility distance for a lamp matrix sign with 18-in. character heights) (2). If the message is too long for drivers to read at normal speeds, it is likely that some drivers will slow down to be able to read the message, affecting the traffic flow (1). In general, the message length should be reduced as much as possible without losing the message intent (1). The message length can be reduced by the use of alternative phrases or appropriate abbreviations and the removal of redundant and unimportant information. Phases: Dudek (1) reports that research has shown drivers have difficulty reading messages that are on more than two phases. Because either the first phase or the second phase may be read first by a passing driver, each phase should make sense by itself. This is accomplished by keeping compatible information units in the same phase. In addition, portions of two different information units should not be displayed on a single line because it is confusing to drivers and increases reading time (1). Design Issues The legibility distance for a CMS is affected by a number of factors. If the sign is placed off to the side of the roadway rather than directly over the travel lanes, additional sight distance is required (1) because a driver's field of view is assumed to be between 10° right and left of head-on. Proffitt and Wade (4) support rotating the CMS 5° to 10° toward the roadway to increase the amount of time that roadside signs are at an optimal reading angle. However, conflicting ideas exist regarding the assumed angular range of the legibility distance. This distance is also affected by lighting conditions, sun position, vertical curvature, horizontal curvature, spot obstructions, rain, fog, and trucks in the traffic stream (1). If the legibility distance of the sign is reduced, then the time that the driver has to read the sign is reduced, necessitating a reduction in the number of information units contained on the sign. Cross References Key Components of Sight Distance, 5-2 Presentation to Maximize Visibility and Legibility, 19-4 Changeable Message Signs, 13-6 Composing a Message to Maximize Comprehension, 19-8 When to Use Changeable Message Signs, 19-2 Displaying Messages with Dynamic Characteristics, 19-10 Key References 1. Dudek, C.L. (2004). Changeable Message Sign Operation and Messaging Handbook. (FHWA-OP-03-070). College Station: Texas Transportation Institute. 2. Dudek, C.L. (1992). Guidelines on the Use and Operation of Changeable Message Signs . (FHWA-TX-92-1232-9). College Station: Texas Transportation Institute. 3. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways . Washington, DC. 4. Proffitt, D.R., and Wade, M.M. (1998). Creating Effective Variable Message Signs: Human Factors Issues. (VTRC 98-CR31). Charlottesville: Virginia Transportation Research Council. 19-7
