An examination of the impact of the increase in the speed limit in 1987 to 65 mph on rural interstate highways in Illinois was conducted. Autoregressive integrated moving average (ARIMA) time series intervention analyses using monthly speed data collected by the Illinois Department of Transportation for 15 rural interstate highway segments were used to examine changes in (a) 85th-percentile speeds and speed variances for cars and large trucks (i.e., vehicles greater than 24 ft in length) and (b) car-truck speed differentials. ARIMA analyses were also used to examine the changes in accident frequency, accident rates, and proportion of car-truck accidents to all accidents. Consideration was given both to all accidents and to fatal and injury accidents only. Using may 1987 as the point of intervention (i.e., the increase in the speed limit), the ARIMA analyses are based on time series consisting of 52 preintervention months and 15 postintervention months. Results were obtained for four individual highway segments and for all 15 segments collectively. An increase of 4.0 Mph in the 85th-percentile speed for cars was detected at the 95% confidence level. No change was found in the 85th-percentile speed for trucks or in the speed variance for either cars or trucks. An increase of 2.8 mph in the car-truck speed differential was found at the 80% confidence level. Although the frequency of all accidents increased by 14% following the speed limit change, no statistically significant change in the frequency of fatal and injury accidents only was found. No change was detected in the rate for all accidents, but an increase of 18.5% Was detected in the rate for fatal and injury accidents only. Significant reductions were found for both the rate of car-truck fatal and injury accidents and the proportion of car-truck fatal and injury accidents to all fatal and injury accidents.
Samenvatting