Enhancement of Vehicle Speed Estimation with Single Loop Detectors

Abstract

Traffic data from single loop detectors are a dominant data source widely used in traffic operation centers and traveler information systems. Speed estimation from single loop detectors is based mainly on occupancy data, a conversion factor from occupancy to density (which is related to vehicle length), and the assumed relationship between flow, speed, and density. The discrepancy between the speed estimated with single loops and the speed measured directly from double loops was investigated. It was found that the inaccuracy of speed estimation done with single loops was caused mainly by the irregular behavior of vehicle pace. Under congested or unstable traffic conditions, the distribution of vehicle pace within a given time interval often exhibits a large variance accompanied by a strong skewness. Accuracy in speed estimation can be improved by computing occupancy in a different way—by using median vehicle passage time over the detector—instead of mean vehicle passage time, often used in the conventional method. The performance of the enhanced speed estimation method is very encouraging. The use of the median vehicle passage time reduces the skewness of pace data.