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1 INTRODUCTION Highway engineers are constantly redesigning and rebuilding roadways to meet higher standards, provide safer highways and increase mobility. For the last forty years this has included designing and building roadways that are more forgiving when a driver inadvertently encroaches onto the roadside. There are, however, many impediments that keep highway designers from achieving the desired design and operational goals of safety and mobility including the need to operate, maintain, and improve a vast highway system with limited resources. Consequently, highway engineers are often required to make incremental improvements over time and make difficult trade-offs between cost, safety and mobility. Modeling tools that enable highway engineers to estimate the cost, effectiveness and benefit of making various safety improvements are essential in making these decisions and trade-offs. Single vehicle run-off-road (ROR) traffic crashes account for almost one-third of the deaths and serious injuries each year on the Nation's highways. In 2007, for example, there were 41,059 fatal crashes, of which 37% were ROR crashes. [NHTSA08] Perhaps more alarming, 46% of all fatal crashes were a result of rollovers or collisions with fixed objects even though these account for only 19% of all crashes indicating these types of crashes may be proportionally more severe. Historically, models developed to explain the relationship between ROR crashes and roadside features and fixed objects, such as utility poles, traffic sign posts, trees, guardrail, median barriers and side slopes, have been categorized as either crash-based or encroachment- based models. Each of these approaches has developed independent of the other, with accompanying software, research and publications to assist users with the application. The primary objective of this research was to develop crash-based methods to model the frequency and severity of ROR crashes for potential inclusion in the AASHTO Highway Safety Manual (HSM). The secondary objective was to coordinate the developed crash-based methods with the available encroachment-based methods implemented within the third version of the Roadside Safety Analysis Program (RSAPv3) to allow each methodology to benefit from future developments to either approach. This report describes the background, the research approach, the resulting ROR crash predictive methods and presents a draft chapter for consideration by AASHTO for publication in the HSM. The draft HSM chapter is presented in Appendix A. This report also details the recommended modifications to the RSAPv3 encroachment probability model to allow for coordination with this effort. The recommended modifications are summarized in Appendix B.
