An evaluation of inductance loop detectors for speed measurement accuracy

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 best wire type, shortest lag time condition, and most accurate speed detection scenario. Single stranded wire and multiconductor wire inductance loops were compared for accuracy of speed measurement. The multiconductor wire inductance loop speed trap measured a speed difference from the infrared light sensor speed trap of only 0.139 mph (0.224 km/h). On the other hand, the single stranded wire speed trap measured a speed difference of about 6 mph (10 km/h). Clearly, the best method for obtaining accurate speeds with inductance loops utilizes multiconductor cable loops. The lag time from the presence of a vehicle at an inductance loop to the actual detection of a vehicle varies. As the lag time varies so does the accuracy of speed measurement. Vehicle size, vehicle speed, detector type, detector sensitivity, and inductance loop wire type all affect the lag time. The shortest lag times were found to occur when using the Naztec Card Rack detector. The multiconductor cable loops also had shorter lag times than did the single stranded wire loops. Measuring accurate vehicle speeds with inductance loop detector speed traps is dependent upon the vehicle mix, vehicle speeds. detector type, and detector sensitivity settings. While using a 9 meter (30 foot) trap, the speed difference between an inductance loop speed trap and an infrared light sensor speed trap was found to be very small for most cases. As speeds increased, the speed difference also increased, Finally, the Naztec Card Rack detector was found to consistently measure the lowest speed differences for most conditions.