Quoting from one of the links provided:
"Traffic engineering research has determined that speed limits should be established according to the 85th percentile of free flowing traffic. This means the limit should be set at a speed that 85 percent of people are driving at or below."
This used to be either part of the law here in Georgia as far as establishing speed limits, or was part of the official procedures, and I'm pretty sure other states followed this also. Evidently it's not being followed anymore here in Georgia (law or not) since the speed limits, especially on local 4 lane roads, seem to be entirely arbitrary and are routinely set at a maximum of 45mph. Driving on 4 lanes with little surrounding buildings or houses, the speed limit would probably be 60mph if they followed the 85th percentile rule. The cops love to frequent these roads since it's like shooting fish in a barrel, it's just about impossible not to exceed the speed limit unless you keep your eyes glued to the speedometer (unsafe in itself).