Abstract: A system and method is provided for preempting the signal lights of an intersection. A magnetic heading of an emergency or other vehicle is measured using a mobile unit and adjusted to account for the magnetic interference of the vehicle. The magnetic heading is transmitted to a base unit installed at the intersection. Upon receiving a verified magnetic heading from a vehicle, the base unit preempts the normal operation of the signal lights to present a green light or other preferred display to the emergency vehicle Additionally the vehicle has a transmitter operating at a high frequency, which limits the range of the transmission. Thus, only the intersection to be preempted receives the transmission from the mobile unit. Also, a tester is provided to evaluate the function of the base unit and the mobile unit without interfering with the normal operation of the traffic signal. Also, the location, type and direction of other vehicles can be tracked even if the signal lights are not preempted.