Crash warning system

A crash warning system for motor vehicles is described, wherein each vehicle is equipped with a transmitter (T) activated by a crash sensor (S, I), preferably an inertia switch, to emit a signal when the vehicle is in collision, and a receiver (R) capable of detecting signals emitted from other vehicles to warn the driver of the existence of a collision in the vicinity. Preferably the signal is a radio signal, and directional receiving antennae (5) ensure that only receivers (R) in vehicles approaching the crash site are activated. The unit may be permanently installed in the vehicle, or may be portable for use in one of a plurality of vehicles by a driver.