The AASHTO Highway Safety Manual (HSM), published in 2010, represents the culmination of 10 years of research and development by an international team of safety experts, academics, and practitioners. As a tool, the HSM allows planners, designers and traffic engineers to evaluate the safety impacts of decisions throughout the project development process on crash frequency and crash severity. The science-based approaches and tools in the HSM add value to the project development process by explicitly facilitating consideration of safety.
Whether the project purpose is safety-related or not, every project can benefit from applying the HSM to the development and evaluation of alternatives. Project decisions are based on full evaluation of costs, right-of-way, traffic operations, environmental factors, and safety. Prior to publication of the HSM, the area of safety lacked a common, science-based, and reliable means of quantification – which is what the HSM offers.
The Integrating the HSM into the Highway Project Development Process guide provides information for state and local practitioners on how to integrate the HSM into their project development process. Each section provides an overview of some of the implementation opportunities for the HSM during each stage of the project development process. The HSM, integrated into agency processes and considerations, will support regional, state, and national fatality reduction goals alongside the goals of mobility, the environment, and other competing needs.
Transportation planning legislation is increasingly emphasizing the importance of safety in transportation planning. When agencies are considering implementing or modifying policies, the HSM provides the ability to assess anticipated changes in crash frequency or severity, allowing explicit consideration of the safety impacts in addition to potential traffic operations and/or economic impacts. For example, if an agency is considering an access management policy on all arterial roadways throughout the community, the HSM provides crash modification factors that quantify the change in crash frequency or severity associated with changing driveway density. Therefore, safety can be a performance measure along with traffic operations and economic impacts. In terms of corridor-specific plans, the HSM can also assist with refinements to the plan by allowing planners and engineers to estimate the change in safety performance across different concepts and approaches considered for a corridor. For example, the HSM can be used to assess the influence of the type and frequency of intersections, driveways, parking, or median types on crash frequency for an urban or suburban arterial.
Within the typical project development process and during environmental analysis, agencies can apply the HSM to include quantitative safety in alternatives development and analysis. The HSM provides methods for agencies to objectively define locations or projects for which the potential for safety improvement is indeed significant or not. With adoption of tools and methods in the HSM, agencies can incorporate the historic safety performance of the existing road into their designation of project type and support the identification of likely reasonable alternatives. Furthermore, agencies can apply the HSM to support explicit consideration of quantitative safety during alternatives development and analysis. In the event that agencies select an alternative that does not have the highest predicted safety performance (e.g., because environmental or other impacts were greater for the particular geometric configuration), agencies can use the HSM to identify mitigating strategies to improve safety performance for the selected alternative.
With the scientific and the predictive method in the HSM, the designer is now able to perform safety performance-based design. (In this guide, the “predictive method” refers to the 18-step process outlined in the HSM. In summary these steps are select and apply the appropriate safety performance function and crash modification factors, apply a calibration factor if available, and apply the empirical Bayes method, if appropriate.) For example, the designer can assess the safety impact of a design parameter, evaluate the impact of design exceptions on safety performance, and review implemented projects to evaluate impacts of design criteria. The science-based human factors fundamentals in the HSM allow the designer to assess the interactions of the road user with the highway and evaluate design solutions based on user abilities and limitations.
The HSM does not require agencies to select a particular solution purely because it has the lowest associated crash frequency or severity. However, the tools in the HSM allow agencies to review a selected alternative (that may not have had the lowest associated crashes or severity) and evaluate opportunities to reduce the associated crash frequency or severity.
In the day-to-day operation and management of the transportation system, agencies are responsible for providing a safe and efficient transportation system for users. Within these programs, projects and activities the HSM offers data-driven and science-based methods and tools to supplement system monitoring, identification of opportunities for improvement and assess safety impacts of operations and maintenance activities.
Typical applications include:
- Identifying safety performance measures across the system.
- Identifying typical locations in a geographical region that may particularly benefit from systemic treatments.
- Identifying and assessing changes in safety performance for different operational and site conditions.
- Evaluating and quantifying the impact of treatments, policies and programs on the safety performance of corridors, road segments, intersections, groups of treatments, or the road-way network.
- Inform and improve maintenance policies and priorities.
- Assessing tradeoffs between funding maintenance improvements to such areas as pave-ments, roadside facilities, and bridge facilities.
A number of analysis tools and supporting developments are available to support implementation and use of the HSM in the project development process. These tools include, but are not limited to, AASHTOWare SafetyAnalyst, the IHSDM, the FHWA CMF clearinghouse, and guides FHWA developed to support implementation of the HSM (for example, training and additional resources). Links to these and other resources are available at the AASHTO Highway Safety Manual web site, the FHWA Office of Safety HSM web site, and the TRB Highway Safety Performance Committee web site.