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Signal Progression Impacts on Transit Buses as Travel Time Probes

This paper examines the growing use of transit buses as traffic probe vehicles for obtaining arterial travel times. Such fleet vehicles inherently behave differently than the private vehicle population. The fleet vehicle data capture the traffic conditions, but they also capture behavior unique to the fleet, e.g., servicing passengers at a bus stop. There are several strategies used in conventional practice to eliminate these biases that occur in the vicinity of the bus stop. The authors originally sought to demonstrate the benefits of using a perception sensor to identify ambient traffic conditions around the bus and correct for transit operations. Indeed, this approach does a better job than the existing methods at segmenting transit operations from traffic delays in the vicinity of the bus. However, the research uncovered a fundamental issue impacting almost all systems that use buses as arterial travel time probes: the fact that transit operations inevitably pull the bus out of the traffic signal progression no matter what corrections are made locally at the bus stop. The impact at subsequent traffic signals far downstream of a bus stop can be much larger than the local effects at the bus stop itself. This point is an important finding for any system that seeks to use transit vehicle probes to estimate the private vehicle travel times. To date the literature has made little consideration of the signal progression biases relative to the private vehicles that occur far beyond the bus stop. Finally, though the focus the present work is buses, the basic approach should apply to other fleets used as travel time probes as well.