Of course, as many commentards have pointed out, the system is open to abuse; but to do that you would have to, if your victim chooses, commit perjury in a court of law. The maximum penalty for this 7 years in prison, so the risk is probably not worth it for most sensible people.
I'm slightly alarmed at the negative comments here, it seems that any enforcement of laws you don't agree with, and are likely to break, is immediately an attempt at setting up a police state!
Speeding is a very petty offence, which most of us commit on a daily basis (myself included), but speeding where people (and their children/cats/dogs etc.) live is very anti social and unnecessary.
Many people who speed are premeditated speeders, they set off on a journey knowing they haven't enough time to complete it if they drive within the speed limits; but their own time is more important than other people's safety.
There are also speeders who just seem to be oblivious to all speed limits; I'm forever stuck behind twats who drive along the 60mph road to the village where I live at 45mph, then continue to drive at 45mph when the speed limit changes to 30mph and race off into the distance.
Whatever the reason for speeding, the only system that actually forces people to obey is average speed cameras because it virtually guarantees conviction if you don't comply.