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Accident Analyses
Most safety improvements are selected and implemented by first determining the cause and effect relationship between site conditions and accident experience. To assist in these analyses, statewide average accident rates are developed for various accident characteristics. Rates are calculated by severity of accidents (fatal, injury, and property damage only), collision type (right angle, left turn, rear end), truck involvement, etc. When average daily traffic volumes are not readily available to calculate a rate, statewide percentages are used. Typically, statewide percentages are used for comparison of accidents occurring on wet pavement, during darkness, involving alcohol impaired drivers and trucks.
Highway Section Evaluation
Accident information pertaining to a specific highway section is compiled in graphical form (see Sample Line Diagram) and in tabular form (See Accident Study Worksheet). The first provides information about individual accidents, while the second is a summary of accident rates for
Sample Line Diagram
Shows Key Accident Information by Log Mile
the location being studied in comparison to the statewide rates for similar state-maintained highway sections. The statewide average accident rates are determined for various types of highways classified by their principle characteristics, e.g., level of access control, urban/rural designation, number of lanes, and divided or non-divided. Rates are calculated for these groupings over a three-year period. To determine if a specific section of highway is a candidate for improvement(s), its accident rates, by type over a three-year period, are compared to the statewide average accident rates for state highways in the same
Accident Study Worksheet
category. If the differences are significant, accident patterns are analyzed to determine what measures are appropriate to reduce accident types that exceed the statewide average rate.
Intersection Analyses
Intersection accident information is also compiled in tabular form (see Accident Data Summary) and graphical form (see Collision Diagram at right). Because there are countless combinations of intersection configuration and traffic control devices, it is not practical to classify intersections by categories as is done for highway sections. For this reason, statewide average rates for intersections are not compiled. Intersections are prioritized according to their rate and severity index, from high rate to low, for analysis by SHA engineers: for the annual Highway Safety Improvement Program.
Severity index rates (weighted severity of accident) are also developed to account for the seriousness of accidents at a particular location.
Before/After Studies
After improvements are implemented, accident information is again analyzed to determine if the improvements were effective in reducing accidents. Based on these studies, accident reduction factors are developed that are used to predict how certain measures will reduce specific types of accidents. By applying a monetary value to each accident reduced, the benefits of an improvement can be compared to the costs. The benefit/cost ratio is an important criterion that SHA uses to screen and prioritize potential improvements.
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