Safety performance functions reflect the complex relationship between exposure, usually measured in annual average daily traffic, and accident count for a unit of road section over a unit of time. One of the main uses of the safety performance functions is to identify locations that experience more accidents than expected, thus exhibiting a potential for accident reduction. Overrepresentation in the number of accidents above the expected or normal threshold predicted by the safety performance function is only one of many indicators of a potential for accident reduction. Accident type, severity, road condition, spatial distribution of accidents, and lighting conditions are only a few of the many important symptoms of the accident problem. Two methodologies are introduced for identification of locations with potential for accident reduction: direct diagnostics and continuous pattern recognition analysis. Use of these methodologies revealed that existence of accident patterns susceptible to correction may or may not be accompanied by the overrepresentation in accident frequency detected by the safety performance functions.
Abstract