Quantifying Loop Detector Sensitivity and Correcting Detection Problems on Freeways
Publication: Journal of Transportation Engineering
Volume 138, Issue 7
Abstract
Loop detectors are the most commonly used vehicle detector for freeway management. A loop detector consists of a physical loop of wire embedded in the pavement connected to a sensor located in a nearby cabinet. The sensor detects the presence or absence of vehicles over the loop and typically allows a user to manually select the sensitivity level of operation to accommodate for a wide range of responsiveness from the physical loop. In conventional practice, however, it is difficult to know the physical loop’s responsiveness, which makes selecting the appropriate sensitivity level difficult. If the sensitivity and responsiveness are poorly matched it will degrade the detector’s data and the performance of applications that use the data, including: traffic management, control, and traveler information. To resolve this often overlooked problem, this paper presents an algorithm to assess how well a loop detector’s sensitivity is set by calculating the daily median on-time from the data reported by the loop detector. The algorithm can be incorporated into conventional controller software or run off-line. The result can be used both to correct the detector on-times for an inappropriate sensitivity setting in software (e.g., through a multiplicative correction factor) and to trigger an alarm to dispatch a technician to adjust the hardware sensitivity. Plotting the daily median on-time over months or years can show how the detector performance evolves. The approach is then transposed to dual-loop detectors to identify and correct for inaccurate spacing between the paired detectors. Finally, the methodology is evaluated by comparing the loop detector speeds against the concurrent velocities from a GPS-equipped probe vehicle. Although the focus of this paper is on loop detectors, with only minor modification the algorithm should also be applicable to other detector technologies that emulate loop detector operation, e.g., side-fire microwave radar.
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Acknowledgments
This material is based upon work supported in part by NEXTRANS the USDOT Region V Regional University Transportation Center and by the California PATH (Partners for Advanced Highways and Transit) Program of the University of California, in cooperation with the State of California Business, Transportation and Housing Agency, Department of Transportation. The Contents of this report reflect the views of the authors who are responsible for the facts and accuracy of the data and results presented in this paper. The contents do not necessarily reflect the official views or policies of the State of California. This report does not constitute a standard, specification, or regulation.
The authors are grateful for the help of the Ohio Department of Transportation in facilitating this research.
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© 2012. American Society of Civil Engineers.
History
Received: Nov 11, 2010
Accepted: Nov 4, 2011
Published online: Nov 17, 2011
Published in print: Jul 1, 2012
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