A method of operating a traffic sensor to define ranges of centers of traffic lanes from the traffic sensor is described. The method comprises a) providing a set of lane center variables representing the ranges of the centers of the traffic lanes from the traffic sensor; b) initializing each lane center variable in the set of lane center variables to have an associated starting range value; and then, c) updating the set of lane center variables by, for each vehicle in a plurality of vehicles, i) detecting the vehicle, ii) determining an associated lane center variable having an associated lane center range value closest to the vehicle; iii) estimating a vehicle displacement from the associated lane center range value, and iv) calculating a new lane center range value for the associated lane centre variable using the associated lane center range value and the vehicle displacement.