Changes over time in the relationship between road accidents and factors influencing them: The case of Norway.

This paper compares the results found in successive accident prediction models developed at the national level for Norway. Over time, the models have become more comprehensive in terms of the roads and the variables included in them. It is found that traffic volume has consistently had the strongest association with the number of accidents. It explains nearly all the systematic variation in the number of accidents. The second most important variable has consistently been the speed limit of 50 km/h, which indicates an urban area (the default speed limit in urban areas in Norway is 50 km/h). This variable has become less important over time. Motorways (freeways) have consistently had a lower accident rate than other roads. The mean number of accidents per road section declined considerably from 1986 to 89 to 2010-15. Systematic variation in the number of accidents between road sections was greatly reduced. At present, the variation in the annual number of accidents between road sections is mostly random.