Highway Safety Manual
The Highway Safety Manual (HSM) provides analytical tools and techniques for quantifying the potential effects on crashes as a result of decisions made in planning, design, operations, and maintenance. There is no such thing as absolute safety. There is risk in all highway transportation.
A universal objective is to reduce the number and severity of crashes within the limits of available resources, science, and technology, while meeting legislatively mandated priorities. The information in the HSM is provided to assist agencies in their effort to integrate safety into their decision-making processes. Specifically, the HSM is written for practitioners at the state, county, metropolitan planning organization (MPO), or local level. The HSM’s intended users have an understanding of the transportation safety field through experience, education, or both.
This knowledge base includes
- Familiarity with the general principles and practice of transportation safety;
- Familiarity with basic statistical procedures and interpretation of results; and
- Suitable competence to exercise sound traffic safety and operational engineering judgment.
The users and professionals described above include, but are not limited to, transportation planners, highway designers, traffic engineers, and other transportation professionals who make discretionary road planning, design, and operational decisions. The HSM is intended to be a resource document that is used nationwide to help transportation professionals conduct safety analyses in a technically sound and consistent manner, thereby improving decisions made based on safety performance.
Documentation used, developed, compiled, or collected for analyses conducted in connection with the HSM may be protected under Federal law (23 USC 409). The HSM is neither intended to be, nor does it establish, a legal standard of care for users or professionals as to the information contained herein. No standard of conduct or any duty toward the public or any person shall be created or imposed by the publication and use or nonuse of the HSM.
The HSM does not supersede publications such as the U.S. DOT FHWA’s Manual on Uniform Traffic Control Devices (MUTCD), Association of American State Highway Transportation Officials’ (AASHTO’s) “Green Book” titled
A Policy on Geometric Design of Highways and Streets, or other AASHTO and agency guidelines, manuals, and policies. If conflicts arise between these publications and the HSM, the previously established publications should be given the weight they would otherwise be entitled if in accordance with sound engineering judgment. The HSM may provide needed justification for an exception from previously established publications.
The new techniques and knowledge in the HSM reflect the evolution in safety analysis from descriptive methods to quantitative, predictive analyses.
What are descriptive analyses?
Traditional descriptive analyses include methods such as frequency, crash rate, and equivalent property damage only (EPDO), which summarize in different forms one or more of the following: the history of crash occurrence, type, or severity at a crash site.
What are quantitative predictive analyses?
Quantitative predictive analyses are used to calculate an expected number and severity of crashes at sites with similar geometric and operational characteristics for one or more of the following: existing conditions, future conditions, or roadway design alternatives.
What is the difference?
Descriptive analyses focus on summarizing and quantifying information about crashes that have occurred at a site (i.e.,summarizing historic crash data in different forms). Predictive analyses focus on estimating the expected average number and severity of crashes at sites with similar geometric and operational characteristics. The expected and predicted number of crashes by severity can be used for comparisons among different design alternatives.
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