There is a catch which is implicit. If you are up to ten or eleven points, then maybe not so bad, but...
If you rely on something other than the Mk 1 eyeball, you could be stuffed if anyone moves the speed limit a few hundred yards. If you rely on such a device, you will possibly make it more likely that you will be nailed, rather than less. Ok as a second chance reminder, but don't get used to it.
Blunkett