I've heard that that also has to do with the uncertainty of the radars, so that if you fight a ticket where you were flagged going less than 10 over and you insist that you were at the limit there's a good chance you'll win.i speed. cop friends basically say that unless the cop is in a bad mood or something, they won't do anything for 9 mph over, and 10 they'll flash you, 11 you're getting stopped.
Personally I usually go with the flow of traffic. The major highways around here have a limit of 100kph but if they're not jam packed in the morning or evening rush hour (in which case you're lucky if you get up to 50kph) then people generally do about 120kph.