Abstract: The present invention relates to a method for monitoring vehicular traffic flow in a road network 1 in an area served by a mobile telecommunications device network 7 having a call management system 10 provided with a mobile telecommunications device positioning system 11 providing positional data for active mobile telecommunications devices 12. The method comprises capturing geographical positioning data for individual active devices carried aboard vehicles 12 and converting these into probability vectors representing the likelihood of the vehicle having arrived at any of the possible road components 16 of the road network 7 compatible with the geographical positional data. As the vehicle 12 travels along, this process is repeated and new probability vectors constructed based on the probability of any of the available routes between the new probability vector road component position and the immediately preceding probability vector road component position.