Citroen type DS19 automatically guided motor car, 1960. This 1960 Citroen DS19 has been modified by the Road Research Laboratory for experiments on the automatic steering of vehicles by means of a cable buried in the road. Sensors attached to the front of the vehicle detect the magnetic field generated by a current flowing through the cable. The magnitude of the signal depends on how far the car deviates from the cable and this error signal is used to operate the car's steering to bring the car back in line with the cable. The Citroen DS19 has a high-pressure hydraulic system which is used for operating brakes, steering, gear-change and the self-levelling suspension. It was chosen for these experiments because it was one of the few cars whose power-assisted steering can be readily modified for automatic steering.