Improved Dual-Loop Detection System for Collecting Real-Time Truck Data

Abstract

The WSDOT's dual-loop detector's capability of measuring vehicle lengths makes the dualloop detection system a potential real-time truck data source for freight movement study. However, a previous study found the WSDOT dual-loop detection system was not consistently reporting accurate truck volumes. (Truck volumes estimates by basic length category are developed from the vehicle length measurements produced by the dual-loop detectors.) The problems include undercount, over-count, and misclassification.As an extension of the previous study, this research project investigated and identified possible causes of dual-loop miscount and misclassification under un-congested traffic conditions. A quick remedy method was proposed to improve the performance of the dualloop detection system without replacing any part of the current system hardware or software. A new dual-loop algorithm that can tolerate erroneous loop actuation signals was also developed to improve the performance of the WSDOT loop detection system.The improved dual-loop detection system will provide reliable timely truck volume data. WSDOT will benefit from this work by gaining more reliable volume and vehicle classification data without having to resort to any other traffic detection technologies. square-shaped detectors with a dimension of 6 feet in length and 6 feet in width. In recent years, some damaged square-shaped loop detectors were replaced by circle-shaped detectors. LIST OF TABLES Current WSDOT Single-Loop and Dual-Loop AlgorithmAs shown in Figure 1-1, the principle components of an inductive loop detector system include one or more turns of insulated loop wire wound in a shallow slot sawed in the pavement, a lead-in cable from the curbside pull box to the roadside controller cabinet, and a detector electronics unit housed in cabinet. Simply stated, the loop system forms a tuned electrical circuit for which the loop wire is the inductive element. When a vehicle passes over the loop, it decreases the inductance of the loop. This decrease in inductance then actuates the detector electronics output relay or solid state circuit, which, in turn, sends an impulse to the controller unit signifying that it has detected the passage or presence of a vehicle. After the vehicle leaves the loop, the inductance goes back to its normal level, i.e., when there is no vehicle present on a single-loop detector, the detector rests in the "off" state. The detector changes its state from "off" to "on" when a vehicle arrives at the leading edge of the detector and from "on" to "off" when the vehicle Bit 7 -Two vehicles in the dual-loop detector. Calculation aborted, data were not collected for these two vehicles.Bit 6 -Lost vehicle (vehicle hits only one loop). Calculation aborted, data were not collected for these two vehicles.Bit 5 -Bad occupancy ratio between two single-loop detectors that form a dual-loop detector. If the measured on-time difference between the two single-loop detectors is greater than 10% for a vehicle, this bit will be set. Calculat...