A road toll system comprises a vehicle-mounted unit comprising a satellite navigation system implementing a position tracking function; transmitting means for transmitting position or route information to a remote server; and a unique identification device for attaching to the vehicle, and associated with the satellite navigation receiver. The road toll system function is enabled only when the identification device is present. The identification device comprises an electronic device which is adapted to alter by detachment or attempted detachment such as to prevent the road toll system function being enabled. This system increases the security of this type of system and makes fraudulent use of the system increasingly difficult.