a system for preventing collision wherein a radar system is mounted on a vehicle for generating a relative speed signal and a relative distance signal . there are provided a brake signal generating logical circuit responsive to the relative speed signal and the relative distance signal for actuating a brake actuator , first and second comparators for comparing the relative speed signal and the relative distance signal respectively with reference signals , and an information processing signal responsive to the outputs from the first and second comparators for actuating the brake actuator . there is also provided a braking signal releasing circuit for disenabling the brake actuator after the vehicle has stopped .