Here is a good article about it.
There was a hilarious thread (well at least for the readers) posted on our local boards some years ago about a rider who had been ticketed in West Virginia on Rt. 50 where the speed limit went from 50 to 25 (it was at the edge of a small town bypassed by the big road.) The rider argued there was no sign, the officer insisted there was. After getting his reward the rider went back and sure enough there was a sign, completely obstructed by a several branches of a large bush. He took a few pics then decided to help out the next guy by pulling out the Leatherman and cutting the branches so the sign could be seen. The officer happened across this and promptly cited him for littering.