can you do five miles over the speed limit
You added 5-10 mph over the speed limit to actually go the actual speed limit. Then it just became a normal thing to do and as the traffic generally goes 5-10 mph above the speed limit now, it's easier to keep up with the traffic instead of obeying the limit, and the circle goes on forever.
Because many people travel within 5-10 miles per hour of the speed limit, it is socially acceptable because it isn't causing a traffic hazard and most people recognize that. I read this somewhere but cant find the source now, found something that has similar information.