The use of Safety Performance Functions (SPF) helps to identify locations with potential for accident reduction. SPF provide no information, however, related to the nature of the accident occurrence, it only speaks to the magnitude of the problem. Without being able to properly and systematically relate accident frequency and severity to roadway geometrics, traffic control devices, roadside features, roadway condition, driver behaviour or vehicle type it is not possible to develop effective counter-measures. In other words, there can be no effective treatment without accurate diagnosis. Detection of accident patterns on a section of road suggests a presence of an element or elements in the roadway environment which triggered a deviation from a random statistical process in the direction of reduced safety. Identification of such an element always provides a critical clue to accident causality. In many cases the expected or normal proportion of accidents is counterintuitive, which further emphasizes the need for the creation of framework of diagnostic norms for various types of roads. (A) For the covering abstract of the conference see ITRD no 207828. The reprints are also available at the web - http://www.vti.se/pdf/reports/K18APart1.pdf ; http://www.vti.se/pdf/reports/K18APart2.pdf and http://www.vti.se/pdf/reports/K18APart3.pdf.
Abstract