Control devices and methods for a road toll system that is based on on-board units carried by vehicles, comprising: in a marking vehicle detecting a traffic or toll violation of an on-board unit, and, if a violation exists, transmitting a marker to the on-board unit via a DSRC radio interface; in an on-board unit periodically determining, upon receipt of a marker, the position of the unit and broadcasting a position message containing the respective current position; and in the control unit: detecting the vehicle based on at least one of the position messages that are broadcast by the on-board unit.