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Changes in crash risk following re-timing of traffic signal change intervals

Retting, Richard A.; Chapline, Janella F.; Williams, Allan F.
Accident Analysis and Prevention
March 2002

More than 1 million motor vehicle crashes occur annually at signalized intersections in the USA. The principal method used to prevent crashes associated with routine changes in signal indications is employment of a traffic signal change interval — a brief yellow and all-red period that follows the green indication. No universal practice exists for selecting the duration of change intervals, and little is known about the influence of the duration of the change interval on crash risk. The purpose of this study was to estimate potential crash effects of modifying the duration of traffic signal change intervals to conform with values associated with a proposed recommended practice published by the Institute of Transportation Engineers. A sample of 122 intersections was identified and randomly assigned to experimental and control groups. Of 51 eligible experimental sites, 40 (78%) needed signal timing changes. For the 3-year period following implementation of signal timing changes, there was an 8% reduction in reportable crashes at experimental sites relative to those occurring at control sites (P=0.08). For injury crashes, a 12% reduction at experimental sites relative to those occurring at control sites was found (P=0.03). Pedestrian and bicycle crashes at experimental sites decreased 37% (P=0.03) relative to controls. Given these results and the relatively low cost of re-timing traffic signals, modifying the duration of traffic signal change intervals to conform with values associated with the Institute of Transportation Engineers’ proposed recommended practice should be strongly considered by transportation agencies to reduce the frequency of urban motor vehicle crashes.