Evaluating the Safety Impacts of Increased Speed Limits on Freeways in Kansas Using Before-And-After Study Approach

Reza Shirazinejad,Ahmed Al-Bayati,David York,Sunanda Dissanayake

doi:10.3390/su11010119

Reza Shirazinejad, Ahmed Al-Bayati + Show 2 more

Open Access

https://doi.org/10.3390/su11010119

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Abstract

In the summer of 2011, a change in the Kansas laws came into effect, increasing the speed limit on a selected set of freeway sections from 70 mph to 75 mph. Higher speeds were thought to have economic benefits, mostly because the travel time reduction means people reach their destinations more quickly. In this study, the sections where the speed limits remained unchanged, are compared to freeway sections that have been influenced by speed limit increase, to evaluate safety effectiveness. The study utilizes the before-and-after study with comparison group method to assess the safety effects provided in the Highway Safety Manual (HSM). Two crash datasets, obtained by considering three years before and three years after the speed limit increase, were compared in order to evaluate the safety effects of the speed limit change. The crash modification factors (CMFs) were estimated, which showed that there was a 27% increase in total crashes and a 35% increase in fatal and injury crashes across all sections after the speed limit change, and these increases were statistically significant at 95% confidence level. These confounding results show that the speed limit increase has not been beneficial for traffic safety in Kansas, and hence it is important to be cautious in such future situations. Also, additional data have been presented which would be beneficial in identifying and understanding any behavior change in drivers following a speed limit increase.

Highlights

The link between the speed limit and the number of crashes is a topic of interest to vehicle insurance firms and the general public
Based on the steps listed in the methodology section, all sites should be involved in calculations in order to estimate the crash modification factors (CMFs)
The predicted crash frequency in the before period for the first treated site with fatal and injury crashes is as follows according to Equation (2): 33.35 × exp. (−5.975 + 1.492 × ln (0.001 × 17,025)) = 5.82 (Multiple vehicle crashes) 33.35 × exp. (−2.126 + 0.646 × ln (0.001 × 17,025)) = 24.83 (Single vehicle crashes)

Summary

Introduction

The link between the speed limit and the number of crashes is a topic of interest to vehicle insurance firms and the general public. Transportation agencies have a very important role in identifying proper speed limits for all public roadways within their jurisdiction. Speed limits are an important tool for drivers to be notified about the allowable speed for driving on a specific roadway. According to Bill HB 2192, the secretary of transportation in Kansas was allowed to change the speed limit from 70 mph to 75 mph on about 800 miles of interstates, in July 2011. Some criteria that were identified as important attributes when determining the appropriate speed limit value for the selected sites were the roadways classification as rural or urban, the number of vehicles passing on the road, traffic volumes, and geometric characteristics [1]. Geometric characteristics of a road layout include, shoulder type, the degree of curves, median type, median width, number of lanes, lane width, rumble strip type, and cross slope

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