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HFG CHANGEABLE MESSAGE SIGNS Version 1.0 WHEN TO USE CHANGEABLE MESSAGE SIGNS Introduction When to use changeable message signs refers to the general principles regarding the appropriate display of traveler information messages on CMSs. These signs can be used to effectively manage travel, control traffic, identify current and anticipated roadway conditions, and regulate access (1). However, inappropriate application and use can reduce the effectiveness of these signs. Note that the terms "changeable message sign" (CMS), "dynamic message sign" (DMS), and "variable message sign" (VMS) are used interchangeably in the literature to refer to these signs. Because there is no functional distinction between the terms, "changeable message sign" or "CMS" is used throughout this chapter to refer to CMSs, DMSs, and VMSs. Design Guidelines The following guidelines can be used to improve the effectiveness of displaying traveler information with CMSs. When to Use CMSs Examples (adapted from Dudek (4 )) To display essential information about: Random unpredictable incidents such as crashes Temporary, pre-planned activities such as construction Environmental problems such as snow Special event traffic such as for parades Special operational problems such as reversible lanes Recurrent problems such as travel times due to congestion AMBER alerts or emergency security incidents To display messages for less than 2 weeks. In conjunction with other media (e.g., Highway Advisory Radio) if conveying extensive or complex messages. To display up-to-date, real-time information that is accurate and credible. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 19-2
HFG CHANGEABLE MESSAGE SIGNS Version 1.0 Discussion CMSs are an essential part of the driver information system. They are an important link between transportation agencies and the driving public. They allow for the display of time-sensitive or temporary information that affects travel and in many cases requires drivers to take an action (1). It is important that drivers find these messages to be relevant so that they will continue to pay attention to the signs. A field study analyzed by Richards and Dudek (2) showed that CMSs that are operated for long periods with the same message may lose their effectiveness. If drivers begin ignoring a sign, they may not notice or may ignore important roadway information when it is available (3). Johnson (1) also states that drivers tend to ignore messages that are displayed for long periods of time and recommends that safety campaign messages be limited to a few weeks. The content displayed on CMSs is limited by the amount of time that the driver has to read the display. This time is affected by both the legibility distance of the sign and the speed of travel. The legibility distance is influenced by a number of factors including weather conditions (e.g., rain, fog), geography (e.g., hills), and roadway conditions (e.g., the presence of large trucks) (4). CMS reading times are higher than those for static signs because drivers can scan static signs for essential information whereas they must read the entire CMS to understand its message. Static signs also have the advantages of being seen daily and of being uniformly formatted. At highway speeds, the CMS message must be readable in 8 s or less (4). Displaying messages that are longer than this limit can affect traffic flow and sign credibility. Thus, it is recommended that extensive messages be displayed in conjunction with other traveler information media (1). These media can include Highway Advisory Radio (HAR), 511, websites, and commercial radio. Dudek (4) provides additional guidance on message length, the number of information units in a message, and message phrasing. Credibility is an important factor in the use of CMSs. Many factors can cause reduced message credibility including inaccurate, outdated, irrelevant, obvious, repetitive, trivial, or poorly designed messages (4). The accuracy and relevance of information such as travel time are important, because they can be easily checked by drivers. If the information is proven incorrect, sign credibility will suffer. Reduced credibility can cause drivers to distrust the system and ignore the sign. Design Issues There are two schools of thought concerning what to display on a CMS when no unusual conditions exist or when there are no essential messages to present: (1) always display a message on the CMS regardless of whether there is an incident or unusual condition and (2) display messages only when an incident or other situation warrants a message and blank the CMS at all other times. The advantage of displaying a message on the CMS regardless of whether there is an incident is that drivers will know that the CMS is functioning. However, only 10% to 15% of English and French drivers assume the CMS is broken when it is not displaying a message (5). (This result could be caused by the policy in these drivers' jurisdictions of blanking the screen when there are no unusual conditions.) The disadvantage is that drivers may come to ignore the sign entirely if safety campaign or other non-traffic-related messages are displayed when no unusual conditions exist (1). Thus, this guideline recommends displaying a message only when an incident warrants it and a blank CMS at other times. This policy is followed by 77% of transportation agencies surveyed in a 1997 national survey of 26 agencies (1). It also follows the human factors principles of CMS operation: don't tell drivers something they already know and use CMS only when a driver response is required (4). Cross References Determining Appropriate Message Length, 19-6 Composing a Message to Maximize Comprehension, 19-8 Key References 1. Johnson, C.M. (2001). Use of Changeable Message Sign (CMS). FHWA Policy Memorandum response to James A. Cheatham. Retrieved June 19, 2008 from http://www.fhwa.dot.gov/legsregs/directives/policy/pame.htm. 2. Richards, S.H., and Dudek, C.L. (1986). Implementation of work zone speed control measures. Transportation Research Record, 1086, 36-42. 3. Halloin, D.M. (1996). Impediments to the effective use of portable variable message signs at freeway work zones. In C. Dudek (Ed.). Compendium of Graduate Student Papers on Advanced Surface Transportation Systems (pp. Ci-C34). College Station: Texas A&M University. 4. Dudek, C.L. (2004). Changeable Message Sign Operation and Messaging Handbook. (FHWA-OP-03-070). College Station: Texas Transportation Institute. 5. Montoro, L., Lucas, A., and Blanch, M.T. (2004). Specific design parameters: VMS Part I. In The Human Factors of Transport Signs (pp. 185-198). CRC Press. 19-3
