SPEED VARIANCE AND ITS INFLUENCE ON ACCIDENTS

Abstract

Safety should be one of the major factors considered when the speed limit is being selected for a particular stretch of highway. The level of safety on any highway is however related to certain characteristics of the traffic stream and the geometrics of the roadway. The geometric characteristics of a section of highway, e.g., maximum grade, minimum curvature, etc. are based mainly on the design speed. But in many cases, speed limits are posted without adequate consideration given to these characteristics. An important traffic characteristic which has been found to influence safety is speed variance, but currently little is known about the factors that affect variance of vehicle speeds in a traffic stream. The objective of this study was to investigate the traffic engineering factors that influence speed variance and to determine to what extent speed variance affects accidents rates. Detailed analyses were carried out to relate speed variance with posted speed limit, design speeds and other traffic variables. The major factor identified is the difference between the design speed of the highway and the posted speed limit. It was determined that speed variance will be minimum, if the posted speed limit is between 6 and 12 mph lower than the design speed, and outside this range, speed variance increases with increasing difference between the design speed and the posted speed limit. Other findings are: Drivers tend to go at increasing speeds as roadway geometric characteristics improve regardless of the posted speed limit; and accident rates do not necessarily increase with increase in average speed but do increase with increase in speed variance.