A system for controlling vehicle emissions in a geographic area, the system comprising an engine control system for a vehicle having an engine and a control system, wherein the control system comprises: an acknowledgement signal receiving unit to receive acknowledgement signals issued from vehicles within the target area; and a control signal broadcast unit configured to broadcast a control signal including instructions for modification of a vehicle operating parameter to at least one vehicle in the target area, the control signal being selected in dependence on the acknowledgement signals issued from the vehicle within a target area and a previous compliance history for the vehicle whereby the instructions for modification of the vehicle operating parameter are more stringent if the vehicle has not previously complied with an instruction for modification of a vehicle operating parameter, and wherein the engine control system; comprises: a control signal receiving unit configured to receive the control signal, broadcast to the vehicle in the target area, from the control signal broadcast unit in the control system outside the vehicle during engine operation, an engine controller configured to modify the vehicle operating parameter in accordance with the instructions; and an acknowledgement signal sending unit configured to send: an acknowledgement signal indicating any change of the vehicle operating parameter made in respons to the control signal and a history of one or more vehicle operating parameters. an acknowledgement signal receiving unit to receive acknowledgement signals issued from vehicles within the target area; and a control signal broadcast unit configured to broadcast a control signal including instructions for modification of a vehicle operating parameter to at least one vehicle in the target area, the control signal being selected in dependence on the acknowledgement signals issued from the vehicle within a target area and a previous compliance history for the vehicle whereby the instructions for modification of the vehicle operating parameter are more stringent if the vehicle has not previously complied with an instruction for modification of a vehicle operating parameter, and wherein the engine control system; comprises: a control signal receiving unit configured to receive the control signal, broadcast to the vehicle in the target area, from the control signal broadcast unit in the control system outside the vehicle during engine operation, an engine controller configured to modify the vehicle operating parameter in accordance with the instructions; and an acknowledgement signal sending unit configured to send: an acknowledgement signal indicating any change of the vehicle operating parameter made in respons to the control signal and a history of one or more vehicle operating parameters. a control signal receiving unit configured to receive the control signal, broadcast to the vehicle in the target area, from the control signal broadcast unit in the control system outside the vehicle during engine operation, an engine controller configured to modify the vehicle operating parameter in accordance with the instructions; and an acknowledgement signal sending unit configured to send: an acknowledgement signal indicating any change of the vehicle operating parameter made in respons to the control signal and a history of one or more vehicle operating parameters. an acknowledgement signal indicating any change of the vehicle operating parameter made in respons to the control signal and a history of one or more vehicle operating parameters.