The one I've seen were not installed on the toll booths themselves (those are only there when entering or exiting the freeway) but on separate kind of portals a

The one I've seen were not installed on the toll booths themselves (those are only there when entering or exiting the freeway) but on separate kind of portals above the road. A camera records the traffic, a computer extract the plates. Then later you have the same setup, and they can then calculate your average speed between the two. The French name is "radar tronçon".