While it's true that a lot of people speed, I doubt I've ever seen more than about 25% going more than 5 over, and probably no more than 50% going even 1 over (except when I've driven in California, where it seems most everyone speeds everywhere). Around here I5 is usually 3 lanes, and the right two (which are easily the most populated) are the speed limit or under 90% of the time. The left is usually about 5 over, with people making room for the few that push faster than that.
Personally I think people that speed just like to spread that bs in an attempt to rationalize their selfish behavior (again, except in California where absolutely EVERYONE is selfish, so by comparison it's the norm).