Research from Purdue University shows that Bluetooth would be a very good way to track travel time. Bluetooth devices give off unique IDs which could be used to measure speed and movement of pedestrians and vehicles.
But why stop there? Wouldn't it be possible to track people via their Bluetooth signal, if you knew one of their device IDs? Anyway, here's the abstract (thanks, Roland.)
Travel time is one of the most intuitive and widely understood performance measures. However, it is also one of the most difficult performance measures to accurately estimate. Toll tag tracking has demonstrated the utility of tracking electronic fingerprints to estimate link travel time. However, these devices have a small penetration outside of areas served by toll facilities, and the proprietary tag reading equipment is not widely available. This paper reports on tracking of a wide variety of consumer electronics that already contain unique digital fingerprints.