Evaluation of the 55 MPH Speed Limit

Abstract

The purpose of this research was to determine the effect of the 55 mph speed limit on typical measures of speed, compliance of the public to posted limits, accident rates, and anticipated relative gasoline savings on rural primary highways in Indiana. Spot speed data was collected at twelve established Joint Highway Research Project (JHRP) speed stations so that comparisons to previous years could be made. Speeds for 1974 averaged five to ten mph below their 1973 counter part. Observed speeds were statistically lower in 1974 for passenger cars and heavy trucks on all but one class of rural highways. The only exception was for heavy trucks on two-lane highways where no significant reduction in mean speeds was found. An analysis of traffic volumes for the first six months of 1974 indicated that they were 13% less than the projected volumes for 1974 and 6.5% less than the volumes for the first six months of 1973. Accident data was collected on 125 sections of rural primary Indiana highways. Traffic volumes for the sections were used to find accident rates for interstate, other four-lane highways. The rate for the total number of accidents for each class of highway significantly decreased in the first six months of 1974.