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The severity of traffic-related injuries has been studied by many researchers in recent decades. However, previous research has seldom accounted for the effects of curbed outside shoulders on traffic-related injury severity. This study applies the zero-inflated ordered probit (ZIOP) model to evaluate the influences of curbed outside shoulders, speed limit change, as well as other traditional factors on the injury severity of single-vehicle crashes. Crash data from 2003 to 2007 in the Illinois Highway Safety Database were employed in this study. The ZIOP model assumes that injury severity comes from two distinct sources: injury propensity and injury severity when this crash falls into the injury prone category. The modeling results show that on one hand, single-vehicle crashes that occurring on roadways with curbed outside shoulders are more likely to be injury prone. On the other hand, the existence of a curb decreases the likelihood of severe injury if the crash was in the injury prone category. As a result, the marginal effect analysis implies that the presence of curbs is associated with a higher likelihood of no injury and minor injury involved crashes, but a lower likelihood of incapacitating injury and fatality involved crashes. In addition, in the presence of curbed outside shoulders, the change of speed limit adds no significant impact to the injury severity of single-vehicle crashes. Moreover, the modeling results also highlight some interesting effects caused by vehicle type, light and weather conditions, and drivers' characteristics, as well as crash type and location. Through a comprehensive evaluation of the modeling results, the authors find that the ZIOP model performs well relative to the traditional ordered probit (OP) model, and can serve as an alternative in future studies of crash injury severity., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Objective: 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., Methods: 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., Results: 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., Conclusions: 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.