CHAPTER 1

Introduction

Background

Development of the AASHTO Highway Safety Manual (HSM) began from a TRB conference session and a workshop, both held in 1999, that identified the need for a traffic safety document analogous to the role of the Highway Capacity Manual for traffic operations. The first edition of the HSM, often referred to as HSM1, was developed through a series of NCHRP projects and was published by AASHTO in 2010. Predictive methods for freeways and ramps were completed in NCHRP work after publication of HSM1 and were published by AASHTO in 2014 as a supplement to HSM1.

Following publication of HSM1, the AASHTO Highway Traffic Safety Committee, the TRB Committee on Highway Safety Performance, and an NCHRP planning study identified needs for an HSM second edition, referred to as HSM2. A number of individual NCHRP research projects were conducted to develop new material for HSM2, and NCHRP Project 17-71 was initiated in 2015 to integrate the results of these new research projects into a second edition of the HSM. NCHRP Project 17-71 ended during 2020. NCHRP Project 17-71A, “Proposed AASHTO Highway Safety Manual, Second Edition,” was initiated in 2021 to complete the work begun in NCHRP Project 17-71. This final report documents the entire research conducted under NCHRP Project 17-71A to develop and prepare a proposed HSM2 in a format suitable for adoption as an AASHTO publication.

Research Objective and Scope

The objective of this research was to complete the work initiated as part of NCHRP Project 17-71 to develop and prepare a proposed HSM2 in a format suitable for adoption as an AASHTO publication. The proposed HSM2 synthesizes and incorporates relevant ongoing and completed research, including completed NCHRP Project 17-71 deliverables, related documents, and user feedback, to expand the scope and quality of HSM2 to increase its application and improve its usability.

The scope of this project did not involve a complete rewrite of HSM1. Several new chapters were incorporated into the proposed HSM2, but for the most part, existing chapters were updated to incorporate relevant ongoing and completed research, rather than being fully rewritten. NCHRP Project 17-71A did not include as project deliverables the development of spreadsheets or other tools to implement the HSM2 methodologies.

Research Approach

The research in Project 17-71A was conducted in two phases and seven logically sequenced tasks, as discussed in the following.

Phase I—Assemble Resources and Plan Project

• Task 1—Kickoff Meeting and Project Management
• Task 2—Review Materials from NCHRP Project 17-71
• Task 3—Assessment of Research for Potential Incorporation into HSM2
• Task 4—Develop Glossary of Terms and Phrases to be Used and Avoided in HSM2
• Task 5—Prepare Interim Report

Phase II—Produce Proposed HSM2

• Task 6—Execute Approved Phase II Work Plan
• Task 7—Prepare Project Deliverables

In Task 1, the research team performed technical, managerial, and administrative tasks necessary to support Project 17-71A.

In Task 2, the research team conducted a thorough review of the materials from NCHRP Project 17-71 to assess the status of work begun under that project toward the production of HSM2 in a format suitable for adoption as an AASHTO publication. The materials available from NCHRP Project 17-71 included draft chapters, review comments on chapters, responses to those comments, plans for incorporating new material into chapters, and a review of the preceding material by the NCHRP Project 17-71A panel and the AASHTO HSM Steering Committee.

In Task 3, the research team performed an assessment of completed and ongoing research for potential incorporation into HSM2. The research team reviewed and assessed research specifically conducted for HSM application, including projects charged with developing draft material for HSM2 aimed at filling gaps or correcting inconsistencies, and other safety-related research. The research team also reviewed the Phase I interim report from NCHRP Project 17-71, with the applicable tables and plans set forth by the Project 17-71 team to integrate completed and ongoing research as part of their project.

In Task 4, the research team developed a style and usage guide of terms and phrases to be used and avoided in HSM2 to guide its production through to the final draft. The style and usage guide served as an internal tool for the research team as it drafted and revised content for the individual HSM2 chapters so as not to inadvertently create tort liability implications in its content.

In Task 5, the research team summarized the results of Tasks 1, 2, 3, and 4 in an interim report and recommended optional work plans for developing the HSM2 in a format suitable for adoption as an AASHTO publication. The research team submitted the Task 5 interim report and met with the panel to review the interim report and approve a recommended Phase II work plan for execution. At the end of the panel meeting, the research team received approval to proceed with the recommended Phase II work plan.

In Task 6, the research team executed the approved Phase II work plan as presented in the Task 5 interim report.

In Task 7, the research team prepared portions of the final project deliverables, including (a) the proposed HSM2 in an electronic format suitable for transmittal to AASHTO for balloting and eventual publication, (b) a final report documenting the entire research effort, (c) a Microsoft PowerPoint presentation detailing the updated chapter outline for the HSM2 and significant differences between the first and second editions, and (d) a recommended implementation plan.

Outline of Report

This final report presents an overview of the entire research project. The remainder of this report is organized as follows:

• Chapter 2—Review of Materials from NCHRP Project 17-71
• Chapter 3—Assessment of Research for Potential Incorporation into HSM2
• Chapter 4—Development of Style and Usage Guide of Terms and Phrases to Be Used and Avoided in HSM2
• Chapter 5—Modifications and Additions to HSM1 Materials for Use in HSM2
• Chapter 6—Lessons Learned in HSM2 Part C Development
• Chapter 7—Future HSM-Related Research Needs
• References
• Appendix A—Style and Usage Guide of Terms and Phrases to Be Used and Avoided in HSM2
• Appendix B—Calibration of Part C SPFs to a Common State