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

The Washington State Department of Transportation (WSDOT) has a loop detection system on its Greater Seattle freeway network to provide real-time traffic data. The dual-loop detectors installed in the system are used to measure vehicle lengths and then classify each vehicle into one of four categories according to its length. The dual-loop’s capability of measuring vehicle length makes the WSDOT dual-loop detection system a potential real-time truck data source for freight movement study in that truck volume estimates by basic length category can be developed from the vehicle length measurements produced by the dual-loop detectors. However, a previous study found the WSDOT dual-loop detection system was not consistently reporting accurate truck volumes. The problems included significant miscounts and misclassifications. As an extension of the previous study, this research project determined possible causes of dual-loop miscounts and misclassifications under non-forced-flow traffic conditions. A new dual-loop algorithm that can address these error causes and therefore tolerate erroneous loop actuation signals was developed to improve the performance of the WSDOT loop detection system. A quick remedy method was also recommended to improve the performance of the dualloop detection system without replacing any part of the existing system hardware or software. As a byproduct of the research, a laptop-based detector event data collection system (DEDAC) was also developed to help collect loop event data for the research.