Patterns of Single-Vehicle Crashes on Two-Lane Rural Highways in Granada Province, Spain: In-Depth Analysis Through Decision Rules

In Spain, 74% of injury crashes occur on rural two-lane highways. Therefore, one of the strategic priorities detailed in highway safety plans is the specific study of these highways. This study aimed at investigating crash patterns and contributory factors on rural two-lane highways so as to propose specific road safety countermeasures. The analysis method consisted in identifying decision rules extracted from decision trees (DTs). As the traditional method of rule extraction is limited by a DT's structure, some important relationships between variables may not be identified. For this problem to be overcome, an in-depth method for extracting rules from DTs was used. Because the implementation of any corrective road safety measure is constrained by the available resources, the strongest patterns that describe the road safety issue must be extracted. For identification of the strongest rules, a new criterion, lift increase criterion, was defined. Single-vehicle crashes on two-lane rural highways in the province of Granada in Spain were analyzed. Crash data were relative to the 7-year period 2003 to 2009. Rules were obtained by using both gain information and the information gain ratio as splitting criterion. The rules obtained by application of both criteria were consistent and complementary; therefore, the authors recommend the use of both methods to build DTs. Results of the study highlighted several patterns contributing to severe crashes and potentially effective countermeasures. Main patterns were pedestrian crashes, run-off-the-road crashes, run-off-the-road crashes involving powered two-wheelers, crashes involving powered two-wheelers, and crashes at night without illumination.