KTC reviewed existing regulations, guidance, and practices to assess the Kentucky Transportation Cabinet’s warning lights program on highway work vehicles. The Kentucky Revised Statutes categorizes KYTC vehicles as “public safety vehicles” and requires their use of amber lights. Because the use of red can be confused with emergency vehicles, KYTC prohibits the use of red on work vehicles. At the national level, standards for warning light colors do not exist. Rather, AASHTO provides guidance on roadway operations equipment and recommends the use of amber and white lights as the primary warning light colors for highway vehicles. AASHTO guidance also prescribes the use of slow, asynchronous flash frequencies; LED light sources; and placement of lights at high elevations and against solid-colored backgrounds. The MUTCD provides little guidance in terms of warning light specifications, including color preferences. A review of existing state agencies, including DOTs, concluded that amber and white are the primary light colors currently in use across highway operations vehicles.

KTC conducted two external surveys to assess warning light products and practices within the U.S. The first survey reviewed vendor LED products and revealed that LED lights are primarily available in the colors of amber, blue, green, red, and white. Fluorescent yellow-green is not available. The second survey requested state DOT information related to lighting systems’ colors, sources, intensity, and placement; responding agency names and policies; and previous state DOT studies related to warning lights. Survey responses indicated 100 percent use of the color amber (as a color type) as well as use of LED light sources. More than 75 percent of reporting agencies place warning lights on their highway work vehicle’s roof to maximize visibility to motorists. Survey results varied dramatically on the differentiation of warning light colors by vehicle type and the differentiation of light intensities for daytime versus nighttime conditions. KTC researchers recommend the use of amber and white colors for KYTC work vehicles, an asynchronous flashing pattern with slow flash frequencies, and LED bulbs. Other recommendations include placement of warning lights at high elevations on the vehicle, placement of warning lights against a solid-colored background, and investigation on feasibility of yellow-green LED lights.

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© 2015 University of Kentucky, Kentucky Transportation Center

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The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the University of Kentucky, the Kentucky Transportation Center, the Kentucky Transportation Cabinet, the United States Department of Transportation, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. The inclusion of manufacturer names or trade names is for identification purposes and should not be considered an endorsement.