State highway agencies, and others responsible for road systems, do not currently have useful tools for reflecting safety in their decisions concerning freeway and interchange projects. This diminishes the weight placed on safety considerations in these decisions. When difficult choices must be made, greater confidence is often placed on predictions of such factors as cost, operational impacts, and environmental impacts, which are expressed in quantitative terms. Therefore, an effective tool to quantify the safety impacts of proposed projects for consideration in planning, design, and operations decisions is needed.
Safety prediction procedures have been developed for rural two-lane highways, rural multilane highways, and urban and suburban arterials, and these will be included in the First Edition of the Highway Safety Manual (HSM) to be published in 2009/10. Base safety models were developed by the Federal Highway Administration for freeways and interchanges, and these are included in the network screening module of the SafetyAnalyst software and in the Interchange Safety Analysis Tool (ISAT). ISAT was developed as an interim tool to meet immediate needs. The ISAT tool provides crash estimates of three typical interchange configurations (diamond, partial cloverleaf, and full cloverleaf), but has limited capabilities and flexibility. This spreadsheet-based tool can be significantly enhanced to improve the quality of the crash estimates and the tool’s overall usefulness.
Research is needed to develop an enhanced prediction methodology and safety analysis tool (ISAT or a successor) for corridor and site-specific analysis. These results will lead to the development of a new chapter on safety predictive models for freeways and interchanges for the HSM and the documentation for the expansion of the Interactive Highway Safety Design Model (IHSDM).
The objectives of this research were to develop (1) an overall framework for the enhancement of safety prediction methodologies for freeways and interchanges to support decision making for planning, network, corridor analysis, and individual site analysis; (2) safety analytical models and procedures within that framework; (3) models and procedures for a corridor and individual site application tool (e.g., enhanced ISAT or successor); (4) a chapter for the future edition of the HSM; and (5) documentation for inclusion of the models in the IHSDM.
Research completed; final report is pending publication. Note: Content from the final report was included in the HSM First Edition Supplement (2014).