First and foremost speed limits are set based on:
roadway geometry,
sight distance,
roadway use factors,
and speed limit consistency.
If these factors are not limiting, the 85th percentile speed is used to set the speed limit. The 85th percentile speed is the speed at which 85% of vehicles are traveling at or under. It is generally accepted that this speed is considered reasonable for the roadway unless superseded by the factors listed above.
Lowering the posted speed limit does not significantly lower traffic speed, and can lead to unreasonable ticketing for acceptable driving behavior.