This study evaluated the accuracy of truck data collected by dual-loop detectors on Seattle area freeways. The objectives of the study were to 1) quantitatively evaluate the accuracy of a representative sample of dual-loop measurements of vehicle volumes and vehicle classifications on the Seattle FLOW system, 2) identify the types and causes of dual-loop data inaccuracies, and 3) recommend appropriate methods for improving the quality of real-time dual-loop measurements.
One representative station with four dual-loop detectors was selected for an extended error analysis. The detector measurements were compared to ground truth data collected via a video image system. Data for 20-second intervals were compared for peak and off-peak periods. Since there was no documentation on the settings of the dual-loop system clock, an independent analysis was conducted to synchronize the dual-loop and video image systems clocks before analysis.
In general, three dual-loop error types were identified through comparison of dual-loop and video ground-truth data: (1) underestimation of vehicle volumes, (2) incorrect assignment of Bin 3 vehicles to Bin 4, (3) incorrect assignment of Bin 2 vehicles to Bins 1 and 3.
Dual-loop measurement errors such as those described may be due to defects in system hardware, software, or the underlying measurement algorithm. Because hardware errors were virtually eliminated as a factor in the current study, the serious errors that still occurred were most likely due to defects in the underlying dual-loop algorithm or in the implementation program. Therefore, to radically improve the quality of dual-loop data, and thus the quality of real-time truck data on the FLOW system, the Washington State Department of Transportation dual-loop algorithm and its corresponding implementation code should be the emphasis of future research.