[apatil]

I didn't have cases like your example in mind. If you couldn't know that the speed limit would decrease soon, eg you had no idea the speed limit was going to change until you were nearly level with the sign, then presumably the law should require you to slow down promptly but safely. The opt-out button shouldn't be required in that case.

The opt-out button would be for rare situations where you actually need to exceed the speed limit for reasons of health or safety.

I don't want information getting into the hands of powerful people without a very good reason, but I think the safety benefits of universal speed limit compliance with exceptions for health and safety would constitute a very good reason.

In terms of privacy, this idea seems more or less equivalent to traffic cameras that capture images of license plates when drivers fail to comply with traffic signals.