E-ZPass is probably the best thing that's ever happened to paying tolls. Those plastic funnels you toss coins into are a close second. But E-ZPass has a bonus feature. It can be used to track you everywhere you go.
E-ZPass uses RFID. It's cool tech, but hackable and trackable in ways that the companies who use it don't really want anyone to talk about. E-ZPass RFID tags, for instance, can be scanned in plenty of places that aren't tollbooths. A hacker known only as "Puking Monkey" has shown that it happens all the time around the streets of NYC, and talked about his findings at DEFCON.