Development of Crash Modification Factors for Comparing Standard Diamond Interchange to Other Common Interchange Designs

There is no doubt that there is a significant need for improving the explicit role of safety in making decisions on roadway planning, design, and operations. In order to achieve the greatest return on the investment of limited budgets, it is imperative that decisions are made based on the best information regarding the safety implications of various design alternatives and engineering treatments. The NCDOT TIP prioritization process continues to produce many interchange improvement projects, showing that there are significant operational and safety problems at many NC interchanges and that potential cost-effective solutions exist for those problems. One difficulty faced by NCDOT throughout the early project development stages, however, is the lack of information on the safety performance of rival interchange design concepts, leading to choices based on factors other than safety. The Crash Modification Factor (CMF) Clearinghouse, a repository of CMFs from studies all over the world, only contains high quality interchange CMFs (rated 3-star or better) for conversion of a standard diamond to a diverging diamond or to a roundabout interchange. The purpose of this project is to help NCDOT make optimum decisions on interchange design concepts. The objective of this research is to develop a set of CMFs for comparing standard diamond interchange to other fairly common interchange designs. Having reliable CMFs for various interchange conversions would allow NCDOT's Traffic Safety Unit to compare the potential safety impacts of common interchange designs and help ensure that the decisions on interchange design concept get closer to the optimum.