How do you apply the Highway Safety Manual to traffic engineering projects?

1 What is the HSM?

The HSM is a document published by the American Association of State Highway and Transportation Officials (AASHTO) in 2010. It provides a collection of methods, tools, and data for quantifying the safety effects of roadway design, operational, and management decisions. The HSM is based on the best available research and practice, and it supports a data-driven and performance-based approach to traffic safety.

2 How is the HSM organized?

The HSM consists of four parts, each covering a different aspect of traffic safety analysis. Part A provides an introduction to the HSM and its principles, concepts, and terminology. Part B covers the predictive method, which estimates the expected crash frequency and severity for a given roadway segment or intersection, based on its geometric and traffic characteristics. Part C covers the crash modification factors (CMFs), which are numerical values that indicate the change in crash frequency or severity due to a specific countermeasure or design feature. Part D covers the economic analysis, which compares the costs and benefits of different safety alternatives.

3 How do you use the HSM for planning?

The HSM can help you identify and prioritize safety issues and opportunities at the network level or the project level. At the network level, you can use the HSM to screen and rank road segments or intersections based on their safety performance, using either the predictive method or the empirical Bayes method. The empirical Bayes method combines the observed crash history with the predicted crash frequency, accounting for the random variation and regression to the mean effects. At the project level, you can use the HSM to assess the safety impacts of different design alternatives, using the CMFs or the safety performance functions (SPFs). The SPFs are mathematical models that relate crash frequency or severity to roadway and traffic variables.

4 How do you use the HSM for design?

The HSM can help you select and implement safety countermeasures or design features that are appropriate for your project context and objectives. You can use the HSM to evaluate the effectiveness of various countermeasures or features, such as lane widths, medians, roundabouts, lighting, signage, etc., using the CMFs or the SPFs. You can also use the HSM to check whether your design meets the safety performance criteria or targets established by your agency or jurisdiction. The HSM provides guidance on how to set and measure safety performance goals and thresholds.

5 How do you use the HSM for evaluation?

The HSM can help you monitor and assess the safety outcomes of your implemented projects, using either the observational before-after method or the cross-sectional method. The observational before-after method compares the crash frequency or severity before and after the implementation of a countermeasure or a design change, adjusting for factors such as traffic volume, crash trends, and seasonal variations. The cross-sectional method compares the crash frequency or severity between sites with and without a countermeasure or a design feature, controlling for confounding factors such as roadway and traffic characteristics.

6 What are the benefits of using the HSM?

The HSM can help you improve your traffic engineering practice by providing a systematic and consistent framework for conducting traffic safety analysis. This tool can enhance your understanding of the relationship between roadway design, operation, and management and crash occurrence and severity, support decision-making with objective and reliable data and methods, demonstrate the value of safety investments, and communicate safety performance and goals to stakeholders and the public.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?