Influence of curbs on traffic crash frequency on high-speed roadways

Curbs are commonly used on roadways for drainage management, access control, and other positive functions. However, curbs may also bring about unfavorable effects on drivers' behavior and vehicle stability when hitting curbs, especially for high-speed roadways. The objective of this article is to investigate whether the presence of curbs along outside shoulders has produced adverse effects on traffic safety on high-speed roadways and whether increasing speed limits has created any further harmful effects.

In this study, the Illinois Highway Safety Database from 2003 to 2007 was selected to evaluate the effects of curbs over traffic safety on 2-lane and 4-lane non-freeways with speed limits of 45, 50, and 55 mph. Wilcoxon/Kruskal-Wallis nonparametric tests were conducted to compare the road-segment crash rates between 3 types of outside shoulders (curbed shoulder, soft flush shoulder, and hard flush shoulder) and 3 speed limits. In addition, the zero-inflated negative binomial models were developed for all of the roadway segments combined, as well as the curbed outside shoulder-only segments. The models were used to estimate the influences of curbed outside shoulder, speed limit level, as well as other roadway characteristics on crash frequency.

It was found that road segments with different types of outside shoulders were from different populations in terms of the distribution of crash rates, so did segments with different speed limits. Further, the crash frequency analysis indicates that the curbed outside shoulders did not induce a higher crash frequency compared to the other 2 types of outside shoulders. In addition, there was no evidence that a decrease in speed limit results in reduction in crash frequencies for road segments with curbed outside shoulders.

The findings of this study suggest that the employment of curbed outside shoulders on high-speed roadways would not pose any significantly harmful effect on the occurrence of crashes, and on high-speed roadways with curbed outside shoulders, reducing the speed limit from 55 to 45 mph would not achieve any safety benefit.