E-ZPass started on the NY Thruway in 1993 and has been expanding over the past decade to most of the toll facilites from Washington DC northward. (The system in Massachusetts is called FastLane but it's the same technology and it's part of the E-ZPass system.) NH is just getting around to it.
The potential for privacy problems is severe, but to their credit I don't think I've ever heard reports of abuses. Besides the possibility of tracking people by tag use, there's the violation tracking issue. If you drive through an E-ZPAss booth in NY with no tag or an invalid tag, a camera takes a picture of your car and they will ask the state DMV to look up the license plate number so they can send you a ticket.
This actually happens, not just for NY violations but for nearby states and provinces as well. In Toronto the 407 toll road has fully automated toll collection, and if you don't have a tag, they send you a bill based on your plate number. (The toll is much lower if you have a tag, so regular users all have tags.) We've gotten bills for our NY car that way, so NY is providing the info to Ontario as well.
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