Scott:
Here in NJ, it's sort of the opposite.
If the sign says "X MPH" everyone assumes they can really do "X + 20 MPH".
I travel on the New Jersey Turnpike at least once a week. The speed limit is 65 MPH. In a recent trip I was jammed up in a gaggle of cars in the middle lane of heavy traffic. We were all traveling around 80 to 85 MPH. We came upon a NJ State Trooper in the slow lane doing about 70 MPH (behind some trucks). No choice.....the traffic dragged us along and we all just sailed past him.
If he'd wanted to give out speeding tickets, he would have had to give out over 1000 of them on the spot.