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for the Special Issue on Emerging Commercial Technologies Wireless magnetic sensors for traffic surveillance

Sensys Networks' VDS240 vehicle detection system is a wireless sensor network composed of a collection of 3 " by 3 " by 2 " sensor nodes put in the center of a lane and a 6 " by 4 " by 4 " access point (AP) box placed 15' high on the side of the road. A node measures changes in the earth's magnetic field induced by a vehicle, processes the measurements to detect the vehicle, and transfers the processed data via radio to the AP. The AP combines data from the nodes into information for the local controller or the Traffic Management Center (TMC). An AP communicates via radio directly with up to 96 nodes within a range of 150'; a Repeater extends the range to 1000'. This range makes it suitable to deploy VDS240 networks for traffic counts, stop-bar and advance detection, and measurement of queue lengths on ramps and at intersections, as well as parking guidance and enforcement. VDS240 is self-calibrating, IP-addressable and remotely monitored. Data are not lost because unacknowledged data packets are retransmitted. The accuracy of VDS240 for vehicle counts, speed and occupancy is comparable to that of well-tuned loops. Because the nodes report individual vehicle events, the AP also calculates individual vehicle lengths, speeds and inter-vehicle headways—measurements that can be used for new traffic applications. In July 2007, VDS240 systems were deployed in arterials and freeways in several cities and states, and 30 customer trials were underway in the U.