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Expected Benefits of Safety Analyst

Highway agencies may experience several benefits from using Safety Analyst:

  • Effectiveness of Decision Making: Safety Analyst automates state-of-the-art statistical approaches as described in the Highway Safety Manual, Part B ("Roadway Safety Management Process") to improve the identification and programming of site-specific highway safety improvements. In some cases, Safety Analyst provides improved procedures for functions that highway agencies already perform in an automated fashion. In addition, Safety Analyst automates procedures performed manually by highway agencies.
  • Efficiency of Decision Support: Safety Analyst integrates all parts of the safety management process into a single, modular software package.

Potential benefits of implementing the individual Safety Analyst modules are as follows:

Network Screening

State highway agencies generally have automated procedures for network screening to identify high-crash locations. Typically, these procedures use threshold values of observed crash frequencies or crash rates, at times combined with a crash severity index. Several drawbacks to traditional network screening procedures include:

  • Observed crash data are subject to regression to the mean, because high short-term crash frequencies are likely to decrease and low short-term crash frequencies are likely to increase as a matter of course, even if no improvements are made.
  • The relationship between crash frequency and traffic volume is known to be nonlinear, but procedures based on crash rates treat that relationship as if it were linear.
  • Most existing procedures focus on which sites experienced the most crashes, not which sites could benefit most from a safety improvement.
  • Some existing procedures do not explicitly distinguish between intersection and non-intersection crashes.
  • Most existing procedures do not explicitly address the safety performance of individual interchange ramps.

Research over the last 20 years has developed new measures of effectiveness and new statistical methodologies for network screening to overcome the drawbacks of existing procedures. Safety Analyst implements several of these new network screening methodologies and measures. For example, Safety Analyst uses an Empirical Bayes (EB) approach that combines observed and predicted crash frequencies to provide estimates of the safety performance of specific sites that are not biased by regression to the mean. A new measure of effectiveness implemented within the EB methodology is "excess" crash frequency. Excess crash frequency is a measure of the crash frequency at a given site above and beyond what is expected at the site given its current conditions (e.g., geometrics and traffic volume). The "excess" crash frequency is a measure of the crash frequency that might be reduced if a safety improvement were implemented.

The state-of-the-art network screening approaches implemented within Safety Analyst can help highway agencies make better decisions about where to invest their safety funds.

Diagnosis and Countermeasure Selection

Diagnosis of safety concerns at specific sites is conducted manually by most highway agencies at present. An important step in diagnosis is the preparation of collision diagrams. Some agencies have automated the process of preparing collision diagrams for intersection locations; many agencies, however, still prepare collision diagrams manually. A basic collision diagramming capability is included within Safety Analyst. Safety Analyst can also interface with commercially available collision diagramming software.

The Safety Analyst software assists in the identification of collision types overrepresented at specific locations and investigation of the specific crash patterns that are present. The software serves as an expert system to guide analysts through office and field investigations of particular sites. For example, Safety Analyst generates a list of questions to be answered during a field visit to the site. Answers to the field investigation questions posed by Safety Analyst are entered into the software and used to identify appropriate countermeasures for potential implementation to improve safety. The logic that identifies appropriate countermeasures considers the crash patterns and related site conditions. The user can then select one or more of the recommended countermeasures for further consideration and can add other countermeasures as appropriate.

Automation of these traditionally manual procedures using an expert system approach may benefit highway agencies by providing a comprehensive and thorough approach to diagnosis and countermeasure selection. The suggestions of office and field investigation procedures and lists of candidate countermeasures generated by Safety Analyst will assist users in considering a wide range of potentially effective countermeasures for implementation.

Economic Appraisal and Priority Ranking

Safety Analyst conducts economic appraisals of the costs and safety benefits of countermeasures selected for potential implementation. The economic appraisal results can be used to compare alternative countermeasures for a particular site and to develop improvement priorities across sites. Safety Analyst includes an optimization program capable of selecting a set of safety improvements across sites that maximizes systemwide safety benefits while staying within a specified budget.

Highway agencies currently use a variety of manual and automated methods for conducting economic appraisals of proposed countermeasures. Some current methods may be linked directly to an agency's crash records system, while others consist of spreadsheets into which data must be manually transferred. Safety Analyst provides an approach to economic appraisal consistent with the requirements of FHWA's Highway Safety Improvement Program (HSIP), with data drawn from existing highway agency data files, while providing flexibility for agencies to adapt the process to their own needs and policies. Most highway agencies do not currently use formal optimization tools, but Safety Analyst allows an agency to develop a safety improvement program that provides the greatest safety benefits for the dollars spent.

In economic analyses, Safety Analyst incorporates the most reliable crash modification factors (CMFs) available to represent the safety effectiveness of specific countermeasures, including those CMFs in the first edition of the Highway Safety Manual. Users can also input CMF values from other sources, such as FHWA's CMF Clearinghouse, for use in economic analyses.

Countermeasure Evaluation

Safety Analyst provides the ability to conduct safety evaluations of improvements after they are implemented. The statistical approach to before-and-after evaluations is based on the Empirical Bayes approach and, thus, is able to compensate for regression to the mean. Evaluations use crash and traffic volume data from existing highway agency records together with the same regression relationships between crash frequency and traffic volume used in the network screening tool.

Most highway agencies do not routinely conduct evaluations of implemented countermeasures, and the few evaluations that are conducted are typically not well designed. Safety Analyst provides a tool to make well-designed before-and-after evaluations easy to conduct. This should help highway agencies document the benefits of their safety improvement programs and provide better estimates of the effectiveness of specific countermeasures to use in the programming of future improvements.