At the end of April 1987, Illinois raised the speed limit from 55 to 65 mph on rural interstates and limited-access highways. This paper examines the effects of this change. It applies ARIMA (Auto Regressive Integrated Moving Average) techniques to a monthly time series of accidents, injuries, and fatalities dating from five years before the limit increase to four years after. Two types of rural highways are examined; those where the speed limit was raised and those where it remained at 55 mph. The impact of higher limits on mean speeds, speed variance, traffic diversion, traffic generation, speed spillover, and issues of benefits and costs are considered. The findings suggest the higher limit led to 300 additional accidents per month in rural Illinois, with associated increases in deaths and injuries. This impact was apparent on both 65 and 55 mph roads. There is some evidence of traffic diversion from 55 to 65 mph highways plus traffic generation and speed spillover. (A)
Abstract