CHAPTER 1

Introduction

Background

The American Association of State Highway and Transportation Officials’ (AASHTO) Highway Safety Manual (HSM), published in 2010, and its 2014 supplement have provided transportation practitioners with a set of predictive methods to analyze roadway safety at all stages of a transportation project—from planning and design to operations and maintenance—as part of an integrated safety management process. These methods help in identifying potential safety issues and implementing appropriate measures intended to reduce the frequency and severity of crashes.

A key component of the HSM was the introduction of safety performance functions (SPFs), which are models used to predict the safety performance of various roadway facilities. Each SPF in the HSM was developed using data from one or more state DOTs. However, the application of these SPFs in other states results in less accurate predictions since driving behaviors, environmental conditions, and crash reporting thresholds vary significantly across the country. To account for these differences, a calibration factor can be applied to the HSM SPFs to account for local conditions. The calibration factor is a multiplicative scalar that is used to adjust predictions from the HSM SPFs to better align SPF predictions with reported safety performance by a given agency. Calibration factors can also be applied for designated regions within a given agency to account for region-specific conditions. Alternatively, DOTs can also develop their own SPFs by collecting crash, roadway, and traffic data from sites within their jurisdiction and estimating their own statistical models. These jurisdiction-specific SPFs can provide more accurate predictions that better match conditions experienced by that agency. However, doing so increases data needs, expertise, and, ultimately, costs. Thus, there is a tradeoff between the calibration of HSM SPFs and the development of jurisdiction-specific SPFs for a given agency.

Synthesis Objective

As the state of practice moves toward more data-driven safety analysis, state DOTs are challenged to calibrate or develop SPFs to meet their needs. The objective of this synthesis is to document the state of practice regarding how states apply and/or customize the methods in the first edition of the HSM, particularly related to state DOT practice on calibration factors and the development of jurisdiction-specific SPFs. The specific items that are documented in this report include:

• Calibration factors and SPFs that states have developed;
• If calibration factors are developed by in-house staff or consultants, what are the respective general levels of effort required;
• What type of analysis tools (e.g., the FHWA Calibrator) are used for the development of calibration factors;

• Factors, including barriers and challenges, and the decision-making process related to the decision to calibrate existing SPFs or develop jurisdiction-specific SPFs;
• What calibration factors are used and how often they are updated;
• Development of region-specific calibration factors or SPFs (e.g., mountain, piedmont, coastal) and how they are used;
• Metrics and methods of validation for calibration factors;
• Adoption of calibration factors from other states and metrics and techniques used to assess transferability; and
• Relevant factors, such as crash reporting threshold, that would affect the applicability of state-specific calibration factors or SPFs to other jurisdictions.

A detailed review of the research literature was performed to identify state DOT practices with respect to SPF calibration and development. In addition, a survey was developed to assess the current state of practice. This survey was sent to all members of the AASHTO committees on Safety, Traffic Engineering, and Design. After completion of the survey, follow-up interviews were performed with select states to better understand their specific practices and challenges with respect to the customization of HSM methods.

Note that at the time that this NCHRP synthesis was prepared, the second edition of the HSM—often referred to as HSM2—was forthcoming but not yet published (the expected publication date is in 2025). This synthesis focuses only on the first edition of the HSM and its supplement.

Report Organization

This synthesis report is organized into the following chapters:

• Chapter 1 – Introduction. This chapter provides background information, the objectives of this synthesis report, and overall report organization.
• Chapter 2 – Literature Review. This chapter summarizes the findings of the literature review, including (a) SPFs in the HSM, (b) calibration and state-specific SPF development procedures, (c) guidance on calibration versus SPF development, and (d) calibration and state-specific SPF developments for individual state DOTs that were found in the literature.
• Chapter 3 – State of Practice. This chapter summarizes the results of the survey of state DOTs.
• Chapter 4 – Case Examples. This chapter summarizes discussions with five state DOTs about their specific practices for applying and/or customizing the methods in the first edition of the HSM, to provide more in-depth information about the methods used and choices made.
• Chapter 5 – Conclusions. This chapter summarizes key observations/trends and suggested opportunities for further research.
• Appendices – Appendix A provides a copy of the full survey that was distributed to state DOTs as a part of this project. Appendix B presents the results of the survey.