Evaluation of safety at freeway interchanges

This research focused on several issues that arise when the Negative Binomial distribution rather than the Poisson distribution (which have been the commonly accepted assumption in analyzing traffic accidents), is found to better fit the accident data.; On the basis of the Negative Binomial distribution, the framework of the rate quality control method was redefined as a basis for the identification of hazardous sites. This produced conceptually more reasonable results than the existing approaches such as the Poisson distribution based rate quality control method, or the Bayes approach.; However, it is sometimes not efficient for traffic engineers to apply this approach since the parameters of the Negative Binomial distribution can not be easily estimated. Therefore, a Normal approximation method to overcome this issue was developed. The Normal approximation method identified the same hazardous sites from a list of two common interchange types found on several freeways in Michigan.; Although the rate quality control method based on the Negative Binomial distribution is an effective technique for the identification of hazardous sites, it has two limitations. First, the selection of reference sites is a matter of judgement. Second, a sufficient number of reference sites with similar characteristics are not always available to assure statistical accuracy. As an alternative, a prediction model method was developed. This method produced results similar to those from the rate quality control method. By using the prediction model method, the conceptual and practical problems associated with the identification of hazardous sites can be reduced. The Generalized Linear Model concept was used to calibrate the accident prediction models.