Role and Application of Accident Modification Factors within Highway Design Process

Abstract

Over the past few years, transportation agencies have placed greater emphasis on incorporating safety into the highway design process. The main assumption is that a highway designed with an explicit attention to safety could significantly reduce the frequency and the severity of crashes. One tool that is rapidly gaining popularity is the use of accident modification factors (AMFs) to estimate the change in safety following implementation of a countermeasure, a significant change in a geometric design element, or a planned construction upgrade of a highway facility. Although AMFs will play an extremely important role in coming years, especially with the forthcoming publication of the Highway Safety Manual (HSM), no document currently exists that addresses how they potentially could be used with the process to design highways. This paper describes the role and application of AMFs in the highway geometric design process. More specifically, it provides guidelines for applying these factors within the preliminary design stage, for assessing design consistency, and for evaluating design exceptions. For each design application, a proposed procedure is described in detail. Finally, an example is provided to illustrate more clearly the descriptions of two of the procedures.