There is also the little matter that , for 20mph limits , there is no specification for accuracy of vehicle speedometers at 20mph , so a driver has no legally specified method of knowing with any degree of accuracy whether he is doing 20 or indeed 24 mph .
The construction & use regs only specify that a speedometer must begin to indicate something at 15 mph , but the specification for accuracy of +10/-0% only begins at 25mph .
Without any legally recognised method of knowing when one is travelling at 20mph , there is a compelling argument against prosecutions for these low speeds . Even using the ACPO guideline of 10% +2mph , a speed of 24mph is right on the edge of the margin , and given no way of accurately measuring these speeds , all such cases must be thrown out .
Even if one were driving at 25mph , ( the lowest speed theoretically enforceable ) at which speed the speedometer could legally indicate 27.5mph , and you then apply the ACPO guideline of +10% +2mph , one could be travelling at 29.5 mph before any action ought to be taken .
The whole notion of 20mph limits , which as they stand can be nothing more than advisory , needs to go to a test case .