A speed limit is the highest speed allowed by law for vehicles on a road.
Speed limits are only peripherally interrelated to the design speed of the road.
In the United States, the design speed is "a selected speed used to establish the various geometric design features of the roadway" according to the 2001 AASHTO Green Book, the highway design manual. It has been changed from previous versions which considered it the "highest safe speed that can be maintained over a specific section of highway when conditions are so positive that the design facial appearance of the highway governs."
The design speed has largely been discredited as a sole basis for establishing a speed limit. Current U.S. standards for design speed derive from outdated, less-capable automotive technology. Also, the design speed of a given roadway is the theoretical maximum safe speed of the roadway's worst feature .The design speed usually underestimates the highest safe speed for a roadway and is therefore considered only a very conservative "first guess" at a limit.
85th percentile rule
An automobile dashboard viewing the speedometer with primary markings in miles per hour.Since the 1950s, United States traffic engineers have been taught the 85th Percentile Rule. The idea is that the speed limit should be set to the speed below which 85% of vehicles are traveling. The 85th percentile closely corresponds to one normal deviation above the mean of a normal distribution.
Every state in the United States statutorily or administratively picks a particular speed for a speed limit cap, meaning that no speed limit in that state may be set higher than the cap. A practical effect of this cap is that nearly every rural roadway in the U.S. has a speed limit that is well below the 85th percentile speed.