The system makes it possible to detect a vehicle on a road and provide two-way communication between the vehicle (4) and a fixed assembly attached to the road, the fixed assembly (1, 6, 7) being capable of communicating via electromagnetic waves with an onboard assembly (3, 5) in the vehicle and equipped with a bidirectional antenna (3), the fixed assembly comprising to this end a short electromagnetic detection circuit (la), in one vehicle traffic direction, and a long communication circuit (1b) forming a bidirectional communication antenna, the communication circuit (1b) being connected in series with the detection circuit (la) and extending along said direction over a greater distance than the detection circuit.