traffic information is gathered by traffic centres and quite frequently it is desired to alert drivers to the presence of a hazard which can be avoided if an early warning can be given . however as most hazards are of local interest then in order to avoid transmitting the early warning nationally , a system is used to transmit the early warning to suitably equipped vehicles using the digital cellular radio network . in view of the fact that the location of the network &# 39 ; s base stations and their contemporaneous coverage areas may be unknown to the traffic centres it is necessary for the traffic centres to relay a description of the geographical area over which an early warning should be given to the control computer of the cellular network so that the control computer can decide which of the base stations provide a combined coverage area most closely matching the description of the geographical area and activate the transmitters of the selected base stations .