Human Factors Guidelines for Road Systems: Second Edition (2012) / Chapter Skim
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Pages 180-194
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When to Use Changeable Message Signs .
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From page 181... ...
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)
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Discussion CMSs are an essential part of the driver information system. They are an important link between transportation agencies and the driving public.
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PRESENTATION TO MAXIMIZE VISIBILITY AND LEGIBILITY Introduction Presentation to maximize visibility and legibility refers to how the photometric and physical characteristics of a CMS can be employed to positively affect readability. Because CMS characters or symbols are typically constructed using a relatively coarse matrix of pixels, the requirements for their visibility and legibility are more demanding than for standard, fixed signs.
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Discussion Contrast ratio: The photometric and physical properties of signs directly affect the legibility of the sign elements. For example, contrast ratios are affected by photometric properties such as luminance, but can be reduced by physical properties such as dirty or scratched protective plexiglass sheeting (1)
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D ETERMINING A PPROPRIATE M ESSAGE L ENGTH 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 me ssage length is extremely important because there is a limited am ount of time to present information to drivers.
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Discussion Information units: The recommendations for the number of information units that are appropriate for display are based on research and operational experience ( 1 )
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C OMPOSING A M ESSAGE TO M AX IMI ZE C OMPREHENSIO N Introduction Composing a CMS message to maximize comprehension refers to message formatting issues that affect driver understanding or reading times. Driver comprehension is important because the message may provide a legitimate safety warning that requires the driver to take an action.
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Discussion Abbreviations: Abbreviations provide the benefit of reduced message length; however, their use is discouraged because they have been found to decrease message comprehension (2) and increase reading times (3)
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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)
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Discussion Only a limited amount of research has been conducted on the dynamic properties of message signs (2)
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CHANGEABLE MESSAGE SIGNS FOR SPEED REDUCTION Introduction CMSs for speed reduction refers to situations in which a reduction in the speed of the traffic flow is desirable due to potential hazards, work zones, adverse weather conditions, incident control, or heavy congestion. Applications that are temporary or variable in nature are the primary candidates for using a speed-reduction CMS.
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From page 192... ...
Discussion General applications: Speed-reduction CMSs are used to reduce speeds during a wide range of events such as potential hazards, adverse weather conditions, traffic incidents, and heavy congestion. When CMSs are provided to reduce driver speeds, compliance is increased when a reason for the reduced speed is displayed (1)
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PRESENTATION OF BILINGUAL INFORMATION Introduction Bilingual information refers to information that is presented in more than one language on CMSs. Drivers spend 10% to 15% more time reading bilingual than monolingual signs if they have more than 1 line in each language (1)
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Discussion Reading response time for one line of relevant text on a two-line bilingual sign is not significantly different than reading response tim e for a one-line monolingual sign ( 1 )
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