Effect of Diverging Diamond Interchange (DDI) on safety performance : application of Empirical Bayes method

Urban population and travel demand have been increasing rapidly. It presents a challenge to transport planners and engineers for alleviating traffic congestion and reducing accident risk. Diverging Diamond Interchange (DDI) is an innovative interchange design, which was first introduced in the United States in 2009, aiming to improve the operation and safety performances of freeway interchanges. DDI can accommodate high and unbalanced traffic volume on the arterial, by reducing the number of traffic signal phases and allowing unobstructed left turns for all traffic directions. DDI has been recognized as a cost-effective solution to congested freeway interchanges. However, few studies have investigated the safety effects of DDI. This paper aims to evaluate the change in crash risk of freeway interchanges after the introduction of DDI, using Empirical Bayes (EB) before and after comparison approach. In this study, the crash data of selected DDIs and traditional diamond interchanges (comparison group) in Utah was collected. The safety performance functions (SPF) were calibrated using the negative binomial model, based on the crash data of 26 comparison sites. Then, the crash modification factors for fatal and injury crash, property damage only (PDO) crash, and overall crash were estimated respectively. Moreover, effects of DDI on pedestrian and bicycle safety were also examined. Results indicated that there were significant crash reductions, regardless of crash severity level, at most of the DDIs under investigation, at the 5% level. For instances, reduction in fatal and injury crash was more remarkable