1. A collision avoidance system for a road vehicle, the system comprising a map storage device for storing road map data distinguishing a plurality of regions of an area that contains a road; a vehicle state detector, configured to detect at least a position part of a current state of the vehicle; a data processor configured to use detected information about a current state of the vehicle to compute a probability density function of future positions of the vehicle in the area, and to compute a correction factor applicable to said probability density function for all positions in a first one of the regions, the data processor determining the correction factor dependent on aggregates of the probability density function in respective ones of said regions.