Accident prediction models for urban and rural carriageways

The SWOV-project Infrastructure and Road Safety aimed to find (mathematical) relations between characteristics of the Dutch road infrastructure and road safety. Such relations are often called accident prediction models (APMs). The SWOV-project developed APMs for distributor roads in The Hague region Haaglanden and for provincial roads in the provinces Gelderland and Noord-Holland. This report discusses the APMs for the distributor roads in Haaglanden. This part of the project was carried out in the European project RIPCORD-ISEREST. In order to develop APMs a database is needed which contains several road characteristics, including the average amount of daily traffic (AADT) and road length. The number of road crashes in a certain period should also be known. For Haaglanden the database Wegkenmerken+ meets these conditions. Wegkenmerken+ is based on the Dutch National Roads Database (NWB). As a consequence single and dual carriageway roads are treated differently. A dual carriageway road is a road on which the driving directions are separated by a physical barrier, so a dual carriageway road consists of two carriageways and each of these carriageways has one driving direction. A single carriageway road consists of only one carriageway and this carriageway can have one or two driving directions. In the NWB, and hence in Wegkenmerken+, the road characteristics are listed per carriageway, so for a dual carriageway road the characteristics are given separately for each carriageway. The accident prediction models are therefore not models for roads, but for carriageways. Examples of the road characteristics listed in Wegkenmerken+ are location (urban or rural), the speed limit, the type of road the carriageway is part of (single or double carriageway) and the number of driving directions. These characteristics are used to define the following carriaway types for which APMs are developed: - carriageways of distributor roads inside urban areas; - carriageways of distributor roads outside urban areas; - carriageways of dual carriageway distributor roads inside urban areas, with a speed limit of 50 km/h, one lane in each driving direction; - carriageways of single carriageway distributor roads inside urban areas, with a speed limit of 50 km/h, two lanes and two driving directions. Two model forms are tested. The fit of the models is extensively checked by conducting several statistical tests on the deviance, the parameter estimates and the standardized deviance residuals. The conclusion of these tests was that all models fit the data reasonably well. The developed APMs were compared to each other. The following conclusions could be drawn: - for AADT is less than about 25000, carriageways inside urban areas generally have a lower crash rate (number of crashes per motor vehicle kilometre) than carriageways outside urban areas; - carriageways with a speed limit of 50 km/h or 80 km/h and one driving direction have a lower crash rate than carriageways with the same speed limit but with two driving directions; - the average crash rate of urban carriageways with a speed limit of 70 km/h is lower than the crash rate of carriageways with a speed limit of 50 km/h; - the average crash rate of rural carriageways with a speed limit of 60 km/h is almost the same as the crash rate of rural carriageways with a speed limit of 80 km/h and two driving directions. Some of these conclusions are counterintuitive. For example, the one which states that urban carriageways with a speed limit of 70 km/h have a lower crash rate than urban carriageways with a speed limit of 50 km/h. However, this conclusion does not state that reducing the speed limit increases the crash rate. For this type of conclusions before and after studies are necessary. Because of the limited size of the database, it was not possible to develop APMs for more detailed carriageway types. Therefore we recommend to collect more data on more roads for further research. This data should not only include characteristics of road segments, but also characteristics of intersections. So far intersections were not considered separately, they were considered as part of carriageways. By developing models for intersections it is possible to investigate the influence of intersection characteristics on the safety of intersections.