Highway traffic flow measurement by passive monitoring of Wi-Fi signals

Motivated by the fact that a significant number of personal mobile devices are carried into vehicles and that the majority of those devices continuously emit Wi-Fi frames, we investigate the feasibility to use these transmissions for road traffic analysis. Background transmissions are emitted by the majority of Wi-Fi-enabled devices by means of so called probe requests. We show by a real-world measurement on an Austrian motorway that a sufficient number of probe requests can be received in order to re-identify devices traveling between two locations and to estimate travel times. Our results show that by proper post-processing of measurement data, meaningful travel time information for the respective highway segment can be derived. We also propose a simple method to protect user privacy by pruning the recorded Wi-Fi device identifiers. Doing so allows us to generate travel time estimates without creating reversible relations between measurement data points and individual devices, which could potentially be linked to user identities. Obviously, using truncated identifiers has an impact on the estimation result. Based on the measurement data at hand we can show that there exists a fundamental trade-off between privacy and accuracy.