This is a subject that has cropped up on more than one forum. If there is a single-carriageway road with a

limit that is not correctly signed, can the police still prosecute someone for exceeding the

of 60 mph, or is no limit in practice enforceable?
If someone is charged with exceeding the 40 limit, but is acquitted because the signage was wrong, then surely to charge them with exceeding the

constitutes a form of double jeopardy. I have never heard of such a case coming to court.
Obviously, even if you believe a limit to be incorrectly signed, you are taking a very risky course deliberately exceeding it, as to get a charge dismissed is likely to cost a lot of time and money.