[jcrawfordor]

I've done some research in this area. Most systems are based on small radar devices mounted to light poles along the freeway which measure vehicle speeds traffic volumes at critical points (some traffic lights use these too now to detect waiting vehicles). A less popular now (because of higher install/maintenance cost) but still in use solution is magnetic induction loops embedded in the freeway lanes, in pairs to allow for speed calculation. The times are then extrapolated from the speeds at critical points (major on/off ramps, junctions, etc). A newer and still somewhat cutting-edge technology is the use of longer-ranch omnidirectional radars that can observe traffic on a road for quite some distance.

One of the really neat things about the modern radar units is that they feed into some software processing that can automatically detect accidents and other types of unusual events and alert authorities.

I'm not saying that there aren't people using bluetooth, but I suspect it's a small minority. I would think that if a municipality wanted to track individual vehicles through an entire section they would be more likely to use LPR, because it's a well established technology and there's a lot of inertia in government purchasing (read: unwillingness to try new tech/manufacturers).

Edit: there's also the confusing issue of vehicles that are tracked by radio transponders - these are going to be voluntary participants though, the obvious groups being people with EZPass type toll transponders and semi trucks with weigh station prepass devices (which are rather similar to the toll system). I wouldn't be surprised if municipalities use this data for traffic observation because it's already being collected for other purposes.

[feld]

Bluetooth is not a small minority. It's a huge business, and Trafficcast is one of the leaders in this. Their product is called BlueTOAD. They're signing contracts with governments left and right.

http://trafficcast.com/news/

MADISON, WI October 18, 2010 - TrafficCast announced it has now finalized agreements with nine leading distributors of traffic signal and control equipment, enabling localized sales services and product support in forty-one states plus the District of Columbia for its innovative BlueTOADTM technologies.

MADISON, WI March 1, 2011 - TrafficCast International, Inc. today announced that Econolite Canada, Inc. will distribute its BlueTOADTM line of products, enabling localized sales services and technical support in the ten provinces and three territories of Canada.