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Some streets in the heart of Washington, D. C., are to become a test site for trying new traffic-control strategies. Surveillance (detection of vehicle presence and estimation of traffic conditions from this information) is an important function of this test site. Traffic parameters compatible with existing surveillance equipment and suitable for traffic control and evaluation have been selected, and their characteristics analyzed. These parameters are volume, occupancy, and queue length. Of these, only queue length uses estimation of individual vehicle motion and therefore is characterized by the greatest uncertainty. Its potential usefulness, however, is great enough to warrant considerable analytical and experimental effort. Equipment required for the surveillance task includes the vehicle detectors, communications, and digital computer. Selection of the detector is influenced by local ordinances, as well as the system requirements; the result was the choice of an inductive loop detector. For the selection of the communications several techniques were compared, and a modified tone-multiplexing approach was chosen. The computer requirements for surveillance include handling high detector input rates, performing high-speed computation and bit manipulation, and providing disk and tape storage for the surveillance-derived data.