North Carolina Department of Transportation (NCDOT) traffic engineers use inductive loop traffic detectors in order to provide efficient signal timing at signalized intersections that do not assign right-of-way under a fixed time division framework. As vehicles travel over the electrified loops, amplifiers in the signal cabinet detect changes in vehicle inductance. The controller uses this traffic demand information transmitted from the amplifiers to modify signal timing on a cycle-by-cycle basis. When the loops are working properly and the signal-timing scheme is appropriate; the result is very efficient operation of an at-grade intersection.
The NCDOT Traffic Survey Unit (TSU) collects the vast majority of traffic data for the Department. Individuals and organizations are demanding ever more current, detailed, and accessible traffic data from the TSU. Unfortunately, limited human resources hamper the ability of the TSU to collect data as often, and in as many locations, as desired. In addition, signalized intersections provide additional challenges of complexity and safety to TSU personnel. The possibility of adding signal detector counts to the mix of available data is very appealing to the TSU.
NCDOT and ITRE have conducted a limited study into the use of advanced detector amplifiers for traffic counting purposes. With the cooperation of Reno A&E, a signal electronics vendor, NCDOT and ITRE tested standard and advanced (dual-output) detector amplifiers in a variety of settings in Wake County.
This research project is designed to see if a) the loop counts are or could be of a reasonable quality to have uses within and beyond the NCDOT Traffic Survey Unit, and b) to incorporate this loop data into signal design, construction, operations, and maintenance procedures.
This report provides a brief primer on traffic operations engineering, since it is important to understand, at least in a general way, how signals are timed and what role inductance loop detection plays. The report continues with some background information regarding the controllers and detectors that are used to implement traffic operations in the field. The document also contains a brief literature review, describing some efforts by other areas in this regard. The report continues with a summary of the user¿s workshop held in 1999. The report then offers a detailed summary of the field experiments conducted in 1999, 2000, and 2001. The paper then provides specific recommendations for implementation, grouped by category. The recommendations apply to both NEMA TS-1 and TS-2 cabinets.
Regarding general recommendations concerning loop accuracy, we identified a high level of congruence between manual counts and the 6' x 6' stretch loops during our field tests. Therefore, our overall recommendation is to begin using stretch (far) loops for traffic counts by rewiring cabinets and installing detector amplifiers with count outputs on an as-needed basis. We identified a substantial variation between the traffic detector data from quadrupoles (tied to detector amplifiers that can detect inductance changes) and the manual count-defined ground truth in several cases, so we do not recommend the use of quadrupoles for counts at this time. As noted in the cabinet wiring recommendations later, the Department does not need to replace every detector amplifier with count-output units; rather, it can simply swap them out as needed for counts. Finally, given that we observed essentially no variation between rhombus, diamond, and square shaped loops during our 2001 field investigation, we recommend that North Carolina retain the use of rectangular (square) 6' x 6' loop shapes.
