Relating the effect of safety measures to expected number of accidents

Traffic engineers have long believed that the effectiveness (percentage reduction in accidents) of a safety measure is greater at locations which had many accidents than at those which had few. That this type of belief has often translated into warrants is evidence of the strength of this belief. Recent research has raised doubts as to whether this phenomenon is real or merely a manifestation of regression-to-the-mean. This paper addresses this issue in the context of an examination of the safety effect of converting 222 intersections in Philadelphia, PA, U.S.A. from two-way to all-way stop control. The results indicate that the conversions were more effective at intersections expected to have many accidents than at the relatively safe intersections. One of the important implications of this finding is that, for measures for which this phenomenon exists, effectiveness cannot be specified as a single accident reduction factor as is currently the practice.