Systems and techniques to determine a primary route between two locations by monitor vehicle speed using transmitting wireless devices. In general, in one implementation, the technique includes: receiving information associated with traffic conditions on a route from a starting location to a destination location; determining a primary route from the starting location to the destination location; determining an average speed of vehicles along portions of the primary route from signals received from wireless transmitters transmitting from the vehicles; identifying one or more delayed portions of the primary route at which the average speed is less than a respective predetermined speed; and displaying the primary route including indicia of the one or more identified delayed portions. The wireless device may include a cellular phone and a personal digital assistant.