"A tool more useful than directions themselves when I was in college at Rice was the Houston Realtime Traffic Map, developed by Texas A&M. This map uses recievers posted at exits from major highways to track Harris County toll road transponders. It calculates the speed for each car as it travels a small section of, say, I-10 and then calcluates the average speed for that section of I-10.
This use of toll transponders is quite valuable for Houston drivers, most of which I believe probably do not have these transponders at all. However, calculating the speed of a car is far from the use most drivers anticipated. Most probably just thought it paid the toll. If this system did something more than just average the data, more privacy concerns would be raised. If the system merely reported back known data to known drivers--their speed on I-10--with no legal penalty, their actions might be different. If this system started generating tickets, something would certainly change. Drivers might forgoe the convenience of the transponders for privacy; they might drive more responsibly.
Toll transponder owners should be aware of the privacy risks associated with transponders, and they should be notified of projects like this one in Houston. My East Coast E-ZPass agreement indemnified E-ZPass from all damages for use by third parties and is unclear at best about what use law enforcement could make. Houston's traffic map shows that your transponder could betray your privacy, showing your location and speed anywhere in the city. "
The comments are valid, but I'd highlight that just because something has the potential to be privacy-invasive does not mean that it will be. Many would say that if the information is properly scrubbed, it is no longer personally identifiable and presents no privacy risk. In any event, operators of these transponder systems should disclose that the data will be used for this purpose so that the more privacy sensitive among us can make the decision of whether to use a tranponder. (But I would guess that the more privacy sensitive don't use transponders because of the other range of potential collateral uses.) Highway operators should also ask themselves whether they could accomplish the same ends by using basic radar technology.