D. Eustace, O. Olufowobi, P.W. Hovey, P. Appiah-Kubi
The impacts of raising speed limits on traffic safety is an area that has generated much research, although a strong consensus has not emerged on the relationship between speed and safety. The Ohio legislature raised speed limits from 65 mi/h to 70 mi/h on 570 miles of rural freeways in Ohio on July 1, 2013 and an additional 398 miles of rural freeways starting on September 29, 2013. The primary goal of the current study was to investigate the safety impacts of this new speed limit using available crash, roadway, and traffic characteristics data. Statewide crash data from January 1, 2010 to December 31, 2015 were obtained from the Highway Safety Information System (HSIS). The study utilized the empirical Bayes (EB) before-after study in the evaluation of the safety effectiveness of the raised speed limit. The intent of the before-after study was to estimate the performance (in terms of crash frequency and severity levels) following the speed limit increase and what the safety performance would have been if there was no increase in speed limit. Safety performance functions (SPFs) were developed for both total crashes and fatal and injury (FI) crashes combined using the negative binomial regression and the SPFs were used to predict the average crash frequency of each of the segments in the observed period. The EB analysis showed that total crashes went down by 24.6% and FI crashes went down by 8.8% for the two years after the speed limit was changed. Therefore, caution should be taken in drawing conclusion from this study because the after period did not meet the minimum of three years recommended by the HSM since the data available for the after period were only for two years. We have received additional 2 years of data and the updated analysis is ongoing.
Keywords: speed limit; empirical Bayes; before-after study; negative binomial; crash frequency; crash severity