Safety Effects of Differential Speed Limits on Rural Interstate Highways

To compare the safety effects of a uniform speed limit (USL) for all vehicles and a differential speed limit (DSL) for cars and heavy trucks, crash, speed, and volume data for rural Interstate highways for the period 1991 through 2000 were obtained from nine states. These states were divided into four policy groups based on the type of speed limit employed during the period: maintenance of a uniform limit only, maintenance of a differential limit only, a change from a uniform to a differential limit, and a change from a differential to a uniform limit. Statistical tests (analysis of variance, Tukey’s test, and Dunnett’s test) and the empirical Bayes method were used to study speed and crash rate changes in the four policy groups. The focus here is only on the statistical tests. The safety effects of DSL and USL were not different within the scope of the study. The mean speed, 85th percentile speed, median speed, and crash rates tended to increase over the 10-year period regardless of which type of speed limit was employed. When all sites within a state were analyzed, temporal differences in these variables were often not significant.

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