Automatic Speed Calibration Methodology for Traffic Monitoring Sites

Abstract

Traffic monitoring sites are installed in highways to collect speed, volume, and classification data to support various planning and intelligent transportation system applications. There is minimal research that addresses speed accuracy and methods to evaluate it without manually collecting vehicle speed data. Accurate speeds are important because the measured speed is used to calculate axle spacing distances and determine the vehicle classification. If the speed measurements are not accurate, the axle spacing distances may not conform to expected values making it difficult to classify the vehicle. This paper identifies values for the drive tandem axle spacing on Federal Highway Administration (FHwA) Scheme F Class 9 vehicles based on truck-manufacturer sales data. A relationship between the drive tandem axle spacing and speed calibration accuracy is verified by collecting and analyzing data from two weigh-in-motion (WIM) sites in Indiana. Based on this relationship, a traffic monitoring lane can be automatically calibrated by looking at historical drive tandem axle spacing measurements. This metric was utilized by the Indiana DOT to identify WIM lanes that required recalibration.