A traffic control system for mobile units comprises a plurality of stationary, passive units having electronic store capacity and transmit and receive facilities. The store of the passive units contains an information code which is unique to the unit and is related to the position of the unit concerned. A mobile unit has a computer with associated store capacity and transmit and receive facilities. When interrogated by a mobile unit, the stationary units supply their information code, enabling the mobile unit to calculate its position. The store of the stationary units, in addition to the information codes of said units, contains stored traffic messages which are supplied together with the information code upon interrogation from the mobile unit. The movement from the mobile unit can be adjusted according to the traffic information received.