Patent US 4,100,529

System for warning vehicle drivers of the proximity of a road hazard. Each road hazard, moving emergency vehicles for example, have associated with them a radio frequency signal transmitter which transmits a pulse length coded signal which is unique for each hazard. Private vehicles have receivers which are responsive to the set radio frequency. When such a vehicle is within range of the road hazard warning transmitter, a light in the vehicle will flash on and off in synchronism with the pulse length coded signal. In addition, this signal drives a sound generator on and off in synchronism with the flashing light producing a coded audio signal. The Radio frequency pulse length coded signal is modulated at an audio frequency in the receiver without disturbing the pulse length code of the radio frequency signal. This then allows a human observer to hear the pulse length code as a coded audio signal.