Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges (2021)

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3 CHAPTER 1: INTRODUCTION 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 cost, operational impacts, and environmental impacts, which are expressed in quantitative terms. Safety prediction procedures have been developed for rural two-lane highways, rural multilane highways, and urban and suburban arterials, and are included in the HSM (Highway, 2010). 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 (Harwood et al., 2010) and in the Interchange Safety Analysis Tool (ISAT) (Torbic et al., 2007). 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. The aforementioned issues led to the conclusion that research was needed to develop a comprehensive safety prediction methodology and safety analysis tool for freeway corridor and site-specific analysis. The research results would lead to the development of a new chapter for the HSM, and documentation to facilitate implementation of the new methodology in the IHSDM. RESEARCH OBJECTIVES The objectives of this research are identified in the following list. ● Develop 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. ● Develop analytical models and procedures within the overall framework. ● Develop a safety analysis tool that automates the framework, models, and procedures. ● Develop a chapter for the future edition of the HSM that documents the methodology. ● Document the models to support their inclusion in the IHSDM. RESEARCH SCOPE To achieve the project objectives, the research scope included consideration of the following freeway and interchange components.

4 ● freeway segment, ● interchange ramp, ● crossroad ramp terminal, and ● freeway speed-change lane. The safety prediction methodology was developed to address a wide range of freeway and interchange conditions. A sample of such conditions is identified in the following list: ● freeway-freeway (i.e., system) and freeway-crossroad (i.e., service) interchanges; ● freeway and interchange facilities in urban and rural areas; and ● crossroad ramp terminals that are signal or stop controlled. The methodology developed in this project can be used to support decision making in the planning and design of freeways and interchanges. The temporal scope of the methodology is also broad to the extent that it includes procedures for evaluating the safety of a segment or corridor for a single year or a specified analysis period consisting of several consecutive years. The methodology supports the following types of design decisions: ● interchange spacing; ● interchange and ramp configuration (e.g., ramp type, location, radius); and ● arrangement of ramps (e.g., successive entrance ramps, weaving section versus collector- distributor road). ORGANIZATION OF THIS REPORT This report presents the results of the research undertaken to develop a safety prediction methodology for freeways and interchanges. Chapter 2 documents the findings from a review of the literature addressing freeway and interchange safety. Chapter 3 presents a framework for safety prediction and summarizes the findings from a series of practitioner interviews. Chapter 4 documents the development of a database suitable for calibrating the predictive models that comprise the predictive method. Chapters 5, 6, and 7 describe the calibration of the predictive models for freeway segments, ramp segments, and crossroad ramp terminals, respectively. Chapter 8 summarizes the steps taken to make the research products ready for implementation. Chapter 9 presents the conclusions and recommendations of the research. Appendix A presents the findings from the practitioner interviews. These interviews identified a range of freeway safety issues and analysis needs. Appendix B describes the activities undertaken to add additional variables to the database assembled for model calibration. Appendix C presents a proposed HSM chapter for freeways and Appendix D presents a proposed HSM chapter for ramps.