Safety Analyst Analytical Tools

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Network Screening Tool
The Network Screening Tool identifies sites with potential for safety improvement. Network-screening algorithms are used to identify locations with potential for safety improvement, for example:
  • Sites with higher-than-expected crash frequencies, which may indicate the presence of safety concerns that are potentially correctable in a cost-effective manner.
  • Sites whose crash frequencies are not higher than expected, given the traffic volumes and other characteristics present at the site, but which nevertheless experience sufficient numbers of crashes that may potentially be improved in a cost-effective manner.

In addition, the network screening tool can identify sites with high crash severities and with high proportions of specific crash or collision types. The network screening algorithms focus on identifying spot locations and short roadway segments with potential for safety improvement, but also include the capability to identify extended route segments. Network screening and all other Safety Analyst algorithms can consider specific crash severity levels (fatalities and serious injuries, fatalities and all injuries, property-damage-only) or all severity levels combined.

Diagnosis Tool
The Diagnosis Tool is used to diagnose the nature of safety problems at specific sites. The diagnosis tool includes a capability to generate crash summary statistics and collision diagrams, to conduct statistical tests for particular sites, to identify predominant collision patterns, and to determine whether those collision patterns represent higher-than-expected frequencies of particular collision types. The diagnosis tool includes a basic collision diagramming capability. The Safety Analyst software can also interface with commercially available collision diagramming software packages, which provide more interactive capabilities and options.

The diagnosis tool guides the user through appropriate office and field investigations to identify particular safety concerns at particular locations. Traditional engineering considerations, as well as a strong human factors component, are used in diagnosis of safety concerns. Output from this step is the identification of specific crash patterns of interest and the development of a list of safety concerns that may potentially be mitigated by countermeasures.

Countermeasure Selection Tool
The Countermeasure Selection Tool assists users in the selection of countermeasures to reduce crash frequency and severity at specific sites. The user can select appropriate countermeasures for a particular site from lists of countermeasures incorporated into the software. The countermeasure selection tool suggests candidate countermeasures based on the type of site, the observed crash patterns, and the specific safety concerns identified in the diagnostic step. The user has the flexibility to select a single countermeasure or multiple countermeasures for a specific site. Where two or more alternative countermeasures are selected by the user, a final choice among them or installed in combination can be made with the economic appraisal and priority-ranking tools.

Economic Appraisal Tool
The Economic Appraisal Tool performs an economic appraisal of a specific countermeasure or several alternative countermeasures for a specific site. Default construction cost estimates for candidate improvements are provided within this tool, but the user has the capability to modify the default estimates based on local experience. The user has the option to select the type of economic appraisal to be performed: cost effectiveness (countermeasure cost per crash reduced), benefit–cost ratio (ratio of monetary benefits to countermeasure costs), or net benefits (monetary benefits minus countermeasure costs). Safety-effectiveness measures (i.e., benefits) are estimated from data on the observed, expected, and predicted crash frequency and severity at the site; the crash patterns identified in the preceding tools; and crash modification factors (CMFs) for specific countermeasures. The CMFs representing the safety effectiveness of particular countermeasures are based on the best available safety research. The analyses include appropriate consideration of the service life of the countermeasure and the time value of money. This tool can perform economic analyses consistent with the requirements of the Federal Highway Safety Improvement Program (HSIP) so that analysis results will be readily acceptable to FHWA for implementation with Federal funds.

Priority Ranking Tool
The Priority Ranking Tool provides a priority ranking of sites and proposed improvement projects based on the benefit and cost estimates determined by the economic appraisal tool. The priority ranking tool can compare the benefits and costs of projects across sites and rank those projects on the basis of cost effectiveness, benefit–cost ratio, net benefits, safety benefits, construction cost, number of total crashes reduced, number of fatal- and severe-injury crashes reduced, and number of fatal- and all-injury crashes reduced. This allows users to fund projects in priority order, with the highest ranked projects being funded first. The priority ranking tool also has the ability to determine an optimal set of projects to maximize net benefits.

Countermeasure Evaluation Tool
The Countermeasure Evaluation Tool provides users with the ability to conduct before-and-after evaluations of implemented safety improvements. Such evaluations are highly desirable to increase knowledge of project effectiveness and supplement or improve the safety-effectiveness measures for improvements available for use in Safety Analyst. This tool is capable of performing before-and-after evaluations using the Empirical Bayes (EB) approach. The EB approach is a statistical technique that compensates for regression to the mean, and also allows for the proper accounting of changes in safety that may be due to changes in other factors, such as traffic volume. This tool also provides users with a capability to evaluate shifts in proportions of collision types. Analyses can be performed to evaluate the effectiveness of individual countermeasures (or combinations of countermeasures) and construction projects. The user also has the option to conduct a benefit–cost analysis to assess the economic benefits of a countermeasure or construction project.