1. A collision avoidance system for a 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; and a data processor configured to use detected information about the 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 the regions, wherein the correction factor is configured to modify a distribution of probability density between the first and a second one of the regions by increasing the probability density in the first one of the regions and decreasing the probability density in the second one of the regions.