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1 1 Introduction 1.1 Background The American Association of State and Highway Transportation Officials (AASHTO) Highway Safety Manual (HSM), 1st Edition (published in 2010) represents the culmination of 10 years of research and development by an international ream of safety experts, academics, and practitioners. The HSM is a powerful tool that can be used to quantify the effects of changes to the roadway environment on safety. The HSM is a potentially transformative document for Departments of Transportation (DOTs) and other agencies responsible for the planning, design, construction, and operation of their highway systems. Under current practices, agency actions are based on results from proven, science-based tools to measure or estimate effects of traffic operations, on a myriad of environmental factors, and on the many aspects of capital and life-cycle costs. However, no proven and accepted tools or methods exist for understanding explicit safety effects. With publication of the HSM, DOTs and other agencies for the first time have access to a proven and vetted science-based means of characterizing the explicit safety effects (such as crash frequency and severity) of the decisions or actions of an agency. The HSM can be used to identify sites with the most potential for crash frequency or severity reduction; identify contributing factors to crashes and mitigation measures; and estimate the potential crash frequency and severity on highway networks, among other uses. The HSM can also be used to measure, estimate, and evaluate roadways in terms of crash frequency and crash severity for corridor studies, traffic studies, environmental impact studies, design analysis, corridor planning studies, and more. The HSM contains the most current and accepted knowledge and practices, and it covers the safety fundamentals, the roadway safety management process, predictive methods, and crash modification factors. The predictive methods focus on roadway segments and intersections for three facility types: rural, two-lane, two-way roads; rural multilane highways; and urban and suburban arterials. Research continues to advance the science of safety and predictive methods for additional facility types will be added as they become available. There is flexibility in the use of the HSM, as there are areas where the analyst has to make a judgment based on several factors including data availability, interpretation, and others. The AASHTO HSM website contains additional information including the Errata to the HSM. 1.2 Using the Highway Safety Manual User Guide The Highway Safety Manual User Guide is a user-friendly document that helps safety analysts use the HSM. The Highway Safety Manual User Guide is a companion document to the HSM and is used as a reference document. It is not a substitution for the HSM or a design guideline for safety projects. It is designed and written primarily for analysts with basic knowledge of the HSM and basic to moderate knowledge of highway safety analysis procedures, but it also contains insights that are useful to all practitioners. The Highway Safety Manual User Guide has three major sections: the HSM overview, integrating the HSM into the project development process, and frequently asked questions. The overview includes the theoretical background of the HSM. The section on integrating the HSM into the project development
SECTION 1 â INTRODUCTION 2 process includes well-designed examples with step-by-step procedures for HSM application. Readers are encouraged to refer to the HSM as well as the following resources: AASHTO HSM website: http://www.highwaysafetymanual.org/Pages/default.aspx FHWA Office of Safety HSM website: http://safety.fhwa.dot.gov/hsm
