Really? I assumed there'd be specs for each limit.
When I complained to my council (with a video showing how ridiculously my car jumped about at 20mph) to the council, I was actually told I shouldn't be going 20mph in a 20 zone. I was told 10mph was a more appropriate speed. I said "change the speed limit signs to 10 then" and she said "we're not allowed." The mind boggles.