ACCURATE ESTIMATION OF TRAVEL-TIMES FROM SINGLE-LOOP DETECTORS

As advanced traveler information systems become increasingly prevalent the importance of accurately estimating link travel times grows. Unfo rtunately, the predominant source of highway traffic information comes from single-loop loop detectors which do not directly measure vehicle speed. The conventional method of estimating speed, and hence travel time, from the single-loop data is to make a common vehicle length ass umption and to use a resulting identity relating density, dow, and spe ed. Hall and Persaud (Transportation Research Record 1232, 9-16, 1989) and Pushkar et al. (Transportation Research Record 1457, 149-157, 199 4) show that these speed estimates are flawed. In this paper we presen t a methodology to estimate link travl times directly from the single- loop loop detector flow and occupancy data without heavy reliance on t he flawed speed calculations. Our methods arise naturally from an intu itive stochastic model of traffic how. We demonstrate by example on da ta collected on I-880 data (Skabardonis et al. Technical Report UCB-IT S-PRR-95-S, Institute of Transportation Studies, University of Califor nia, 1994) that when the loop detector data has a fine resolution (abo ut one second), the single-loop based estimates of travel time can acc urately track the true travel time through many degrees of congestion. Probe vehicle data and double-loop based travel time estimates corrob orate the accuracy of our methods in our examples. (C) 1998 Elsevier S cience Ltd.