EFFECT OF 65-MPH LIMIT ON SPEEDS AND ACCIDENTS

A time series analysis was performed upon fatal accidents, injury accidents, vehicle miles traveled, and vehicle speeds over the five years preceding and one year following the increase in the national Maximum Speed Limit for rural interstate highways during the spring of 1987. In the states raising their limits to 65 mph, speeding on rural interstates increased by 48% and total fatal accidents by 22% over projections based upon prior trends. A 9% increase in speeding and a 1% increase in fataliaties was observed on highways still posted at 55 mph. States with different maximum limits for cars and trucks did not differ from states with uniform limits with respect to changes in the number of truck accidents or the ratio of truck to car accidents. The increase in fatal accidents attributed to raised speed limits in 65 mph states was estimated at approximately 300 per year. A 300 per year increase in fatal accidents occurring in 55 mph states was attributed to increased speeds that occurred coincident with the change in the national maximum limit. The authors speculate that a shift in high speed traffic to rural interstates from other highways may have contributed to the changes occurring in the 65 mph states. The increase in fatalities in 55 mph non-interstate highways in states not raising their limits may have been due, in part, to the absence of such a shift.