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Many agencies, both governmental and private, in the highway transportation field need a system which would automatically identify motor vehicles. The basic concept of such a system requires a vehicle-mounted coded device, a transponder, that uniquely identifies that vehicle. As this vehicle comes within the range of a system interrogator installed along or on the highway, the code is read by this interrogator and the identification of the vehicle is obtained. The ideal transponder is passive and requires no power; has no effect on, and is not affected by the vehicle; does not require physical contact with the interrogator, and requires no action by the vehicle operator. Sensing at a distance (noncontact) involves the detection of some type of energy; either radiated or reflected from the object being monitored. Many different sensing techniques, including magnetic, infrared, optical, sonic, radioactive, radio frequency (RF) transmission, microwaves, have been surveyed, investigated, and analyzed to determine their applicability for an automatic vehicle identification (AVI) system. This analysis has concluded that a low-frequency induction system is presently the most appropriate method for use on the highway environment, Several diverse systems differing in design details have been constructed and tested, utilizing this basic technique. This paper describes the design of these systems as well as the methods that have been proposed and/or developed using the other sensing technologies.