Few, if any. They are a really good idea in so many ways, but Americans regard speed cameras of all sorts as unsporting and invasive.

I used to feel the same way, until some combination of experience with traffic cameras in other countries and maturity changed my mind.

Traffic-stops are dangerous for all concerned, expensive, don't scale, and are rife with the opportunity for profiling/abuse.

To the extent that we can agree on speed limits that delineate the difference between safe and unsafe behavior, average-speed cameras are a great way to ensure that those limits aren't radically exceeded.

There are places, like Highway 20 in Washington State, where the privacy risk is limited. If you pass through Newhalem, you're either going to the mountains or you're going to Mazama, 60 miles away. On that stretch of road, if a car arrives >10% earlier than can possibly be safe, why can't we fine the car's owner?