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The amount of time that a motorist has to read a Dynamic Message Sign (DMS) depends primarily on the size of the letters used to create the message. Generally, the larger the letter, the farther away the message can be read by motorists, and so the longer the amount of time they have to read the sign. For freeway applications, previous DMS legibility studies have determined that 18-inch letters are necessary to provide drivers enough reading time while they are driving at freeway (55 mph or higher) speeds.
Travel speeds on arterial streets are lower than those on freeway facilities, which imply that smaller letters can still provide adequate reading times of a DMS message on an arterial street. Unfortunately, the actual legibility distance of smaller letter heights have not been determined through objective research methods using actual drivers. Based on available research from static signage, the City decided to use a 12-inch letter height on arterial dynamic message signs.
There are many factors that affect the design requirements of a dynamic message sign, including the motorist's vision, sign color, sign illumination intensity, nearby lights, and so on. For lighted dynamic message signs, the most critical factors are:
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