1. A method of collecting, processing and disseminating traffic flow information for a predefined geographic area serviced by a cellular radio system, said method comprising the steps of: a. transmitting, from a plurality of cellular-radio-equipped vehicles travelling in the predefined geographic area to a cellular radio receiver in the cellular radio system, position information periodically identifying the respective positions of said cellular-radio-equipped vehicles relative to predetermined reference positions in said predefined geographic area; b. processing the position information at a central processor in communication with the cellular radio receiver to produce an origin-destination matrix representative of the rates of travel of the cellular-radio-equipped vehicles along predefined route segments in the predefined geographic area; c. transmitting, from a cellular radio transmitter in communication with the central processor to the cellular-radio-equipped vehicles travelling in the predefined geographic area, traffic-flow-rate information derived from the origin-destination matrix.