[ocdtrekkie]

Look up the "85th percentile rule". Traffic engineers actually generally aim to set the speed limit at the speed 85% of drivers are at or under. Sometimes politicians get in the way. For instance, it was requested to raise the speed limits above 55 mph in Chicagoland area highways, and the change was refused, even though the average driving speed on those roads is about 70 already anyways.

Here's a good explanation: http://www.mikeontraffic.com/85th-percentile-speed-explained...