A question of current interest is whether raising the speed limit from 55 mph to 65 mph on rural interstate freeways increased fatal accidents or not. The limit was raised on 1.155 miles of California interstate freeways about June 1, 1987. The speed limit was retained at 55 mph on the remaining 343 miles. Also, the limit was raised on 132 miles of rural "look-alike" freeway about June 1, 1988. This report analyzes fatalities, fatal accidents, and injury accidents before and after the speed limit change. Two years of after data, along with an estimated third year of accidents and travel, were available for the rural interstate analyses; one year, along with an estimated second year, were available for the "look-alike" analyses. Travel increased more on the interstate routes raised to 65 mph than on other state highways. Fatal accident rates had been increasing since 1982, long before the speed limit change, on the rural interseate routes. The rate of increase has remained unaltered since the limit was raised to 65 mph. It is concluded that raising the speed limit to 65 mph on rural interstate routes did not increase fatal and injury accidents, when traffic volume changes and preexisting accident rate trends are accounted for. Also, there were no significant changes in fatal and injury accident rates on interstate routes where the speed limit was retained at 55 mph. There were, on the other hand, apparent increases in fatal accidents on the "look-alike" routes. This should receive further analysis and investigation.
Abstract