Many transportation agencies use re-identification technologies to identify vehicles at multiple points along the roadway as a way to measure travel times and congestion. Examples of these technologies include license plate readers, toll tag transponders, and media access control (MAC) address scanners for Bluetooth devices. Recent advancements have allowed for the detection of unique MAC addresses from Wi-Fi and wireless local area network (WLAN) enabled devices. This paper represents one of

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... e first attempts to measure the fundamental characteristics of Wi-Fi re-identification technology as it applies to transportation data collection. Wi-Fi sampling rates, re-identification rates, range, transmission success rates, and probability of discovery of sensors and mobile devices were measured, and a model of probability of detection is presented. Field tests found that mobile phones routinely experienced significant time gaps between Wi-Fi transmissions. The study recommends that Wi-Fi sensors be deployed at low-volume, low-speed roadways, with sensors positioned near intersections where vehicles are likely to slow or stop. Due to Wi-Fi's relatively low probability of discovery, the technology may produce poor results in applications that require re-identifying vehicles over multiple consecutive sensors.