1. Safety Performance-Based Design
The highway design process has historically centered around applying established design criteria, such as published in the AASHTO Policy on Geometric Design. Highway engineers identify design controls – some of which are predetermined (e.g., terrain, design-year traffic, classification of the road), while others are selected (design level of service, design vehicles, design speed).
Historically, the geometric design criteria have been viewed as the means by which an acceptable level of safety is provided. However, as FHWA and AASHTO both note (Flexibility in Design, AASHTO Guide to Achieving Flexibility in Design) the basis behind geometric criteria includes many factors, (e.g., cost, maintainability, traffic operations) with safety being just one. Moreover, two designers using the same controls and same criteria may create two different alignment and cross-section solutions, with different expected safety performance.
With the HSM, the designer is now able to develop solutions that are based not just on design criteria, but also on quantitative safety performance, as measured by crash frequency and severity alongside operational and project-specific considerations. The designer can also apply science-based human factors fundamentals from Chapter 2 in the HSM to identify and assess design solutions based on user abilities and limitations. As a result, the HSM now allows agencies to perform what is referred to as “safety performance-based design.”
This section discusses three such activities that agencies may implement to explicitly consider safety in design:
- Assess the safety impact of a design parameter.
- Evaluate the impact of design exceptions on safety performance.
- Review implemented projects to evaluate impacts of design criteria.
Assessing safety impact of a design parameter. During the design process, the designer considers options across multiple geometric elements (e.g., lane and shoulder width, curve radii, grade, etc.). The geometric, cross-section, and other project features are selected based on the applicable design criteria, with the primary goal of meeting the project-specific needs in a cost-effective manner. This design process involves choices and tradeoffs. While design manuals and standards are important in this process, balancing quantitative, science-based safety impacts of a design parameter against traffic operations and cost, allows the designer to make overall cost-effective system performance choices. With the HSM tools and approaches, designers can prepare preliminary plans, evaluate their safety performance, refine or adjust one or more elements and reevaluate the performance in a manner similar to balancing cut and fill. The inclusion or exclusion of features such as medians can be tested for safety performance. The predictive method and the CMFs in the HSM provide insight into the impact of individual parameters for particular highway types, as well as individual treatments.
Evaluating the impact of design exceptions on safety performance. Design standards, guidelines and criteria are important; these provide consistency of roadway system deployment and benefits to quality control and ease of construction. Restrictions based on environmental concerns and available right-of-way may require a designer to consider design exceptions (also referred to as deviation(s)) from the established design guidelines and criteria. Design exceptions are common on urban projects and reconstruction projects with extensive constraints. Analysis, decision-making and documentation of the quantitative safety effects of a proposed design exception are among the most significant enhancements the HSM brings to project development. Using the predictive method, the designer can now quantify the impact of a particular exception in terms of crash frequency or severity. This allows for a quantitative assessment of the relative impact of the exception. If a particular exception affects the safety performance of a project negatively, and there is a desire to proceed with the project, CMFs in the HSM may offer options for mitigation. FHWA’s publication Mitigation Strategies for Design Exceptions (FHWA-SA-07-011) describes how such analyses can be done. Finally, the design exception process includes documentation in the form of a report and acceptance by a responsible party such as an agency’s Chief Engineer. Documenting the basis for a design exception decision, including the quantitative safety analysis, is a valuable risk management tool for agencies.
Reviewing implemented projects to evaluate impacts of design criteria. Agencies regularly update geometric design standards, guidelines and criteria to reflect advances in the science of operations, safety and other related fields. The evaluation methods discussed in Chapter 9 of the HSM offer approaches to evaluate projects or bundles of similar projects to quantify the impact of variation in design criteria on safety performance. The HSM discusses different study designs, strengths and limitations of each, and particular considerations. By evaluating completed projects, agencies can update design criteria to incorporate and reflect their relative impact on project safety performance, thus providing continual opportunity for proactive approaches to lower crash or injury risk on the system. There are HSM methods that account for regression-to-the-mean (RTM), and, therefore, provide more accurate estimates.
The tools used to quantify safety performance in the activities described above are similar to those used in alternative identification and analysis (see Section III).
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 improve safety performance by using Part C or Part D of the HSM.
Exhibit 8 continues the hypothetical example from Section III of this Guide for a design exception evaluation.