Soldato
A different scenario, but local to me a bridge which was 50 mph got changed to 30mph on one side only pending structural bridge repairs. A mobile camera conveniently placed about 2 weeks later caught 500 odd people. In the end all 500 had their points and fines squashed and refunded. Whilst i never did find out why the signs were illegal, the police claimed a grace period for the signage change which wasn't true.
Might be something there, though depends on the effort you're willing to go to ultimately. Even when you know "they're" in the wrong, it's made so hard to get justice.
A few years ago in my neck of the woods there was a road that was NSL. There was a downhill stretch then about 1/4 mile later it dropped to 30. Been this way since it was built.
One Sunday the council moved the 30 signs to halfway down the hill & Monday morning the local scamera pratnership had a van at the bottom.
Que the complaints and eventually all fines and points were quashed.
Council claimed that proper planning and consultation had been followed and signage was correct. Court found this not to be the case and actually found the required planning notices had been stapled to a tree, 35 yrd into the wooded area at the side of the road. They were forced to remove the new 30 signs and told to restart the entire process.