On the W&OD trail (major MUP in the DC area), there are several at-grade crossings, with stop signs for the bike path. I've occasionally seen police parked next to the trail, with an officer or two standing outside the car next to the trail......and then have seen cyclists blow right through the sign with the officers standing there.

I think they just got warnings, but I was surprised to see them not only ignore the sign (not slowing down at all), but not to see that the police were stopping cyclists who ran the stop sign.....