New automatic vehicle identification system for detection of traffic without lane discipline

Electronic road toll collection has usually been carried out on the highway using an automatic vehicle identification (AVI) system. At the toll site, physical barriers are set up between lanes so that individual vehicles can be effectively detected. Problems arise, however, in the detection of vehicles on busy city roads where barriers between lanes are not desirable and the traffic does not always maintain lane discipline. A new AVI system using a randomised time division multiplexing (TDM) technique for detecting several vehicles simultaneously entering the detection zone is described. Laboratory experiments carried out for up to eight identification tags at the same time showed that all replies were detected for 900 interrogations. Collisions between replying signals did not occur. The high-speed transmission of serial data has, however, an error rate of 3-7%. Under laboratory conditions, the detection errors were eliminated simply by allowing the tags to reply twice