Quantitative Safety Analyses for Highway Applications (2025)
Chapter: Summary
SUMMARY
Quantitative Safety Analyses for Highway Applications
The first edition of the Highway Safety Manual (HSM) 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 the HSM first edition and were published by AASHTO in 2014 as a supplement to the first edition. Following publication of the first edition of the HSM, often referred to as 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. A number of individual NCHRP research projects were conducted to develop new materials for the HSM, 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, referred to as HSM2. NCHRP Project 17-71 ended during 2020. NCHRP Project 17-71A, “Proposed 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 consideration as an AASHTO publication.
Under NCHRP Project 17-71A, initially the research team conducted a review of the materials from NCHRP Project 17-71 to assess the status of work begun under that project toward the production of HSM2. The materials 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. Next, the research team completed 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, which included plans set forth by the 17-71 team to integrate completed and ongoing research into HSM2. In parallel with these tasks, 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. Then, the research team developed several optional work plans to develop the proposed HSM2 and met with the panel to settle on a Phase II work plan to execute.
This report summarizes the major additions and changes to the HSM, going from the first edition to the second edition. The second edition is not a complete rewrite of HSM1, but rather HSM1 was updated to incorporate relevant ongoing and completed research and to expand the scope and quality of the manual to increase its application and improve its usability. The outline and structure of HSM2 are similar to those of the first edition. The manual still has four
parts (Parts A, B, C, and D). There is an introductory chapter to the manual, which comes before Part A. In the first edition, this introductory chapter was included in Part A.
Six new chapters are incorporated into the second edition. There is one new chapter in Part A (Fundamentals) that addresses pedestrians and bicyclists (Chapter 4). Two new chapters are included in Part B (Roadway Safety Management Process). Chapter 5 addresses an areawide approach to roadway safety management, and Chapter 12 addresses the systemic approach to roadway safety management. Part C (Predictive Methods) includes a new introductory chapter that presents general concepts for applying the Part C Predictive Methods (Chapter 13). Part D (Crash Modification Factors) is new. Rather than presenting information on HSM-approved crash modification factors (CMFs) for roadway segments, intersections, interchanges, special facilities and geometric situations, and road networks, Part D consists of two new chapters on selecting (Chapter 19) and applying (Chapter 20) CMFs that are a key part of the processes and methods described in HSM Parts B and C.
Structurally, all appendices from the first edition have been removed. The information and material that were presented in appendices in the first edition have either been incorporated into the main body of the report or removed from the manual.
HSM1 and HSM2 have each been developed through a series of independently managed research projects. Each project has generally had a separate research contractor and a separate oversight panel. Once these independent research projects were complete or nearly complete, both the HSM1 and HSM2 have had a production contractor to assemble the various independent research project results into a nearly final HSM version that is ready for review and balloting by AASHTO. The challenges in finalizing the HSM2 Part C in NCHRP Project 17-71A indicate a need for closer coordination between the independent research projects conducted for future HSM editions. Several lessons learned in the process of assembling the HSM2 Part C are discussed in the following:
- Research to develop material for inclusion in HSM Part C chapters should not focus solely on developing safety performance functions (SPFs) for specific facility types without also considering what adjustment factors (AFs) can be used with those SPFs.
- Research to develop CMFs intended for use as AFs in HSM Part C procedures should also indicate for which SPFs the new AFs may appropriately be used.
- Each research team developing a new crash prediction method or revising an existing crash prediction method for HSM Part C should be responsible for ensuring that their method is compatible with and can provide results that can be used with and compared to predicted values from existing HSM methods for facility types that constitute likely design alternatives for the facility type addressed by the new or revised method.
- Each new or revised crash prediction method should be based on the same or compatible definitions of crash severity levels, crash and collision types, and crash assignments to roadway segments versus intersections and freeway mainline and speed-change-lane segments versus ramps as the existing HSM methods for facility types that constitute likely design alternatives for the facility type addressed by the new or revised method.
- Each research team that develops revised SPFs intended to replace existing SPFs for specific facility types in HSM Part C should compare the existing and revised SPFs and demonstrate that the revised SPFs provide realistic and reasonable results and are an improvement on the existing SPFs.
- Each research team that develops a predictive method for a facility type not currently addressed in HSM Part C should be responsible for comparing the results of the new predictive method to predicted values from existing HSM methods for facility types that constitute likely design alternatives for the facility type addressed by the new method and demonstrating that the new method provides reasonable results in comparison to those other facility types.
- Each research team should conduct sensitivity analysis of the model output using ranges of the variable inputs and validate results with crash records for various and realistic vehicle and vulnerable user volumes and design element alternatives. Sample problems should be developed with reasonable ranges of input variable values for use in the HSM or other AASHTO or FHWA documents and for developing training material.
There may be valid reasons for research teams developing HSM materials to deviate from some of these suggestions, but this should not be done without consideration of the effect of the decision on the usefulness of the HSM as a whole.
Finally, through the development of the proposed HSM2, several future HSM-related research needs were identified. These research needs may be considered in formulating future HSM-related research programs. It may also be determined that some of these issues or topics are better addressed through training or other types of HSM-related materials, such as a practitioner’s guide.
