One of the major factors that should be considered in selecting the speed limit for a stretch of highway is safety. It is generally accepted that the level of safety depends on certain characteristicsof the traffic stream and the geometric alignment of the highway. However, in many cases speed limits are posted without adequate consideration given to these characteristics. For example, an important traffic characteristic that influences safety is speed variance. Currently, little is known about the factors that affect the variance ofvehicle speeds in a traffic stream. Presented in this paper are theresults of a study that investigated the influence of different traffic engineering factors on speed variance and quantified the relationship between speed variance and accident rates. A major influence on speed variance is the difference between the design speed and theposted speed limit. It was determined that speed variance will approach minimum values if the posted speed limit is between 5 and 10 mph lower than the design speed. Outside this range, speed variance increases with an increasing difference between the design speed and the posted speed limit. It was also found that drivers tend to drive at increasing speeds as the roadway geometric characteristics improve, regardless of the posted speed limit, and that accident rates do not necessarily increase with an increase in average speed but do increase with an increase in speed variance. This paper appears in transportation research record no. 1213, Human performance and highway visibility: design, safety, and methods.
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