NY explained :
I've lost count of the number of times that I've been following other traffic
that is all doing under the speed limit, and yet one of them *still* slams
the anchors on dangerously hard when they see a police car that might be
looking for speeding drivers. Many drivers seem to think that they must brake
even if they are nowhere near exceeding the speed limit.

I think that is likely them braking, because they don't even know what
the speed limit is and suddenly develop a guilty conscience.