An airport safety system is disclosed, comprising surface movement radar for monitoring the movement of aircraft and land vehicles on an airport, said surface movement radar providing a signal input to a computer arranged to identify from the relative motions of aircraft and vehicles detected by the radar in accordance with a pre-programmed set of rules an aircraft at risk of collision. The computer is also programmed to cause transmission by radio of an audible alert signal when a risk is predicted, said radio transmission being at a standard aircraft communication frequency, such as the standard ground communication VHF radio channel. The alert signal can alternatively be transmitted by one of a plurality of message transmitting devices arranged at different locations adjacent to airport runways and taxiways, each message transmitting device comprising a radio transmitter connected to antenna means arranged to radiate a signal within a predetermined area at the location. The radio transmitters operate at standard Marker beacon frequency amplitude modulated by said alert signal. The computer is programmed to identify the transmitting device adjacent to said aircraft at risk of collision and to direct said alert signal to the transmitting device so identified for transmission thereby. In this way, the alert signal could be directed to only the aircraft involved.