During the summer of 2002, the Michigan DOT deployed a prototype variable speed limit (VSL) system on a segment of I-96 in Lansing. The VSL system was comprised of seven trailers with microwave vehicle detectors, dynamic message signs (DMS), remote processing units, solar power, short-range radio communication between trailers, cellular telephone modems for remote data retrieval, and a pager system for law enforcement personnel to determine displayed speed limits.
The seven trailers were deployed at half-mile to one-mile intervals along a work zone on I-96 during a before-and-after study. In the "before" period, the DMS were covered with static 50 mi/hr speed limit signs. In the "after" period, the DMS were uncovered and the VSL system was activated. The system monitored traffic conditions and displayed speed limits ranging from 40 to 70 mi/hr based upon roadway geometry, the prevailing speed, the day of week, and the type of construction underway. For both periods, traffic data were collected from the microwave vehicle detectors and from pneumatic tube vehicle detectors installed by the Michigan DOT.
The prototype VSL system required fairly constant attention from technicians due to device failures, operational problems, and communication issues. Despite this fact, the system operated fairly well. When the system was activated, average speed increased and travel time decreased in the work zone. Speed variance did not appear to be affected by the system. However, limited data indicated that the percentage of high-speed vehicles (e.g., exceeding 60 or 70 mi/hr) decreased during system operation.
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