An evaluation of inductance loop detector lead length and optimal speed trap distance

Abstract

Reducing congestion in our nation's urban areas has become a top priority of state departments of transportation nationwide. The development and construction of freeway management systems have recently begun to help reduce these problems. A major component of freeway management systems is the inductance loop detector. This research effort evaluated the use of inductance loop detectors in a freeway management situation to determine maximum permissible lead lengths and an optimal speed trap distance. Lead lengths were evaluated with five different design vehicles (large and small passenger cars, a pickup truck, a motorcycle, and a high profile truck) with several detector units. Both passenger cars and the pickup truck were always detected with 4000 feet (1220 metres) of lead wire on all combinations of sensitivity level and number of wire turns. The detection of the motorcycle and high profile truck depended upon the sensitivity level and number of wire turns used. The data also indicate that detection of the passenger vehicles may be possible at distances much greater than 4000 feet (1220 metres). An extrapolation of the inductance measurements indicate that detection of passenger vehicles at 8000 feet (2440 metres) is likely, and detections may even be possible at much large distances. Lead lengths of this magnitude give the designer greater flexibility in producing the most functional and cost-effective design. Five different speed trap distances (from 20 to 80 feet, 6 to 24 metres) were evaluated with a variety of detector units. An optimal speed trap distance was not able to be identified, but the research consistently identified that accurate speed measurements cannot be made using different models of detector units. In order for speed measurements to be accurate, the change in inductance required for detection must be very similar between detector units. Identical make and model detector units easily meet this criteria. The error measured for speed traps with identical detector units averaged about 1.5 mph (2.4 km/h) for all speeds, while errors for speed traps with different detector units varied widely from one to 120 mph (1.6 to 194 km/h). Clearly, the best method for obtaining accurate speeds with inductance loop detectors utilizes identical make and model detector units.