On Saturday, I rode past seven of them on my bike. None of the roads were closed. Well, they may have been closed some distance further along, but I followed one road for five miles (accounting for three of the signs) and it wasn't closed.
This happens a lot. The worst signs are the ones they put at the roundabout outside Colchester North train station, which often refer to road closures ten or more miles away. They don't say that though. Once, the road closure was about a hundred yards up the street. There's no way of telling whether a sign affects you or not without ignoring it.
Locals therefore routinely disregard these signs, on the grounds that whoever puts them there has called wolf too often. Non-locals turn around, even though they could be fine if their journey ends before wherever the road closure is. It could be in Cardiff or Exeter for all anyone knows.
Ignoring signs that might, on very rare occasions, conceivably be right annoys people, though, especially once they find out where the road closure is and it's in the next county. Many have therefore taken to sabotaging the signs when this is the case.
Sometimes, the signs are just pushed over:
Other times, they're rotated ninety degrees then turned around so they don't face oncoming traffic:
There are other combinations too. When the sign is in two parts, with the top saying "road ahead closed" and the bottom showing to do a U-turn, the top is flipped down to cover the bottom. Sometimes, the sandbag that's used to hold the frame in place is removed and hung over the sign to cover its message.
I'm sure the people who put the signs up know they're going to be sabotaged, but do it because they'll be sued by the relevant agency if they don't. Ordinary citizens, though, while generally law-abiding, are apt to rebel when faced with something that's plainly stupid. The fact that they're unlikely to be caught makes it only slightly more probable.