Space-Based Passing Time Estimation on a Freeway Using Cell Phones as Traffic Probes

This paper examines the usability of mobile cellular networks to obtain traffic information on a freeway. The question of whether a mobile station (cell phone) can play an acceptable role as a probe for collecting traffic information on a freeway is examined. A space-based approach, wherein the probe vehicles transmit information to roadside devices as they pass through reference points, is exploited rather than a time-based approach, wherein the probe vehicles report information for every specific instant of time. The latter has been of concern to most researchers interested in the use of a mobile cellular network for collecting traffic data. First, a simple analytical model is introduced to address the usability of cell phones as traffic probes and to pinpoint which factors affect the qualification of probe phones when the space-based approach is adopted. Second, simulation experiments are also employed to deal with more realistic traffic conditions as supplementary tools for the analytical model. Finally, the actual traffic data on a freeway was considered to validate the above two hypothesized traffic conditions. The findings show that there are three main factors that affect the qualification of cell phones as a traffic probe: (1) the speed profile of the probe phone in cell coverage; (2) the variability of handoff location where the probe phone switches its jurisdictional cell; and (3) the locational relationship between a reference point and a speed jump (or drop) point in cell coverage.