Ahem, as the old cliche says, it's a limit, not a target!
I beg to disagree... you get a Prius (sorry Prius owners) driving 5 MPH below the posted speed limit on a road perfectly safe to drive on 10 MPH above that limit and 20 car drivers behind getting frustrated this is far more likely to cause an accident. Most speed limits are based on vehicle technologies from the 1960's with commensurate braking distances and handling. In our area 5 MPH above the posted limit is pretty normal and IMHO safe. When you have a vehicle travelling 10 MPH below the normal traffic speed there is a problem. I would far prefer concentration of enforcement on vehicles running red lights or veering around while texting both of which are far more dangerous than marginally exceeding posted limits.