Abstract: A warning device D is fitted to an emergency vehicle and is arranged when deactivated, to transmit a radio signal carrying an audio warning,. this radio signal being transmitted repeatedly at successively different carrier frequencies, corresponding to the channels available in the local area. The VHF/FM radio receiver of any vehicle within range will tune to the signal of one of these carrier frequencies, as being identical or close to, but stronger than, the channel selected by the driver, so that the audio warning will be delivered to the driver.