Analytical Tools for Loop Detectors and Traffic Monitoring Systems

This paper presents two useful analytical tools developed in the course of evaluating the performance of a freeway management system (FMS). The first calibrates speed measurements from loop detectors using only data reported from a given detector and the assumed free flow speed, e.g., the posted speed limit. The approach finds a correction that sets the daily median speed to the expected free flow value, and it proved able to correct for changing performance over time even in the presence of noisy measurements. The results at both dual-loop and single-loop detectors are evaluated using a GPS equipped probe vehicle. The second tool is conceptually similar to the first, using the weekday median value from each week to present clearly on a single plot the time-series evolution over several years of delay, volume, or other system performance measures. Without this weekly median, typical daily variability makes it difficult to present in a single plot the trends over more than a few weeks.