Interchange configurations safety comparison tool

The Interstate System is a fully access controlled surface transportation network connecting diverse geographical areas for the movement of people and goods. It is of national interest to preserve and enhance this system. Service interchanges are critical facilities linking State and Local roads to the Interstate System. When the need arises to insert a new interchange or modify an existing interchange, the request is presented in the Interchange Justification Report (IJR). Although many IJRs are submitted annually, the types of interchanges proposed are limited. Improvements in traffic operation due to proposed interchanges are usually well-described based on traffic analysis results. However, the safety performance of proposed interchanges is often not well substantiated due to a lack of safety modeling tools. Historic data indicates that high-traffic facilities like interchanges exhibit stable traffic crash patterns over time, meaning they should be predictable. Although many factors are known to induce traffic crashes, the safety community hasn’t fully grasped how to properly quantify those factors. Decomposing an interchange into simple components and adding up the components’ annual crash predictions generally doesn’t yield results that match field data. The Federal Highway Administration (FHWA) developed the Interchange Safety Analysis Tool (ISAT) and supported the development of its enhanced version, ISATe. Both of these tools decompose interchanges into simple components for analysis. The FHWA recently completed a study treating the service interchange as a whole and using input data typically available in the planning phase to predict the safety performance of planned service interchanges. It includes the safety predictions of eight interchange configurations representing 78 percent of all interchanges considered in IJRs reviewed by FHWA. This paper presents the functionalities of this tool and explains how the tool may be used. This tool gives the IJR reviewers a consistent methodology for assessing the safety performance of proposed service interchange projects.