Trafficmaster's SmartNav package uses kit fitted in users' cars and a call/data centre. The car is linked to TrafficMaster by GPRS (and a voice channel for the driver). Drivers can call in to TrafficMaster and ask for a route plan from the call centre; the call centre has real-time traffic data as well as ordinary mapping and locations.
Instructions are delivered by a dashboard speaker or optional screen. The car system has GPS satellite nav, and tells TrafficMaster where it goes in real time.
TrafficMaster sells the info in bulk to the Department for Transport.