Power and Exponential Functions Relating Accidents to Traffic and Rain. Calibration on a French Network.

Relations between the occurrence of road accidents, traffic and rainfall conditions are valuable in setting safety objectives for traffic management, and in assessing the safety impacts of new traffic management systems, prior to their implementation. Based on traffic, road accidents and rain data collected over one year, on a French urban motorway network, a set of safety performance functions were estimated; each of them provides the accident risk per vehicle-kilometer for a certain type of accident, according to the occurrence of rain, and to the level of a traffic variable (average speed, occupancy, percentage of tailgating...). Analyses were carried out separately by lane and for two types of accidents: single-vehicle accidents and multiple-vehicle accidents.The relationships, although statistically significant, have yet to be validated by the treatment of another set of accidents