Smartphones with Wi-Fi enabled periodically transmit Wi-Fi messages, even when not associated to a network. In one 12-hour trial on a busy road (average daily traffic count 37,000 according to the state DOT), 7,000 unique devices were detected by a single road-side monitoring station, or about 1 device for every 5 vehicles.In this paper, we describe a system for passively tracking unmodified smartphones, based on such Wi-Fi detections. This system uses only common, off-the-shelf access point hardware to both collect and deliver detections. Thus, in addition to high detection rates, it potentially offers very low equipment and installation cost.However, the long range and sparse nature of our opportunistically collected Wi-Fi transmissions presents a significant localization challenge. We propose a trajectory estimation method based on Viterbi's algorithm which takes second-by-second detections of a moving device as input, and produces the most likely spatio-temporal path taken. In addition, we present several methods that prompt passing devices to send additional messages, increasing detection rates an use signal-strength for improved accuracy.Based on our experimental evaluation from one 9-month deployment and several single-day deployments, passive WiFi tracking detects a large fraction of passing smartphones, and produces high-accuracy trajectory estimates.

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A real-time travel time estimation system is demonstrated that uses Wi-Fi monitors deployed along surface streets to capture Wi-Fi packets from smartphones in passing vehicles. Travel time is estimated based on observations of the same vehicle by two or more monitors.

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