If you're speeding and caught by a camera then it's already too late, you may slow back down to the speed limit but I bet most people will continue to speed knowing they've already been caught and angry at that very fact.
If you're speeding and you see a marked police car, officer or some other kind of marking suggesting that there is a chance you could get caught, you will more than likely slow down to avoid being caught and continue to drive slower because they're in the area and you haven't been caught yet... Maybe they could sponsor people to drive marked traffic police cars to deter people who speed?