A speed limit is the maximum speed allowable for vehicle by law on a road. The Speed limits are only peripherally connected to the intend speed of the road. In the United States, the plan speed is "a chosen speed used to determine the different geometric design features of the roadway" according to the 2001 AASHTO Green Book of the highway design manual. It has been altered from previous versions which considered it the "maximum secure speed that can be maintained over a definite section of highway when conditions are so positive that the design features of the highway govern."
The design speed has basically been discredited as an only basis for establishing a speed limit. The Current U.S. standards for design speed derive from outdated, less-capable automotive technology. In addition, the design speed of a given roadway is the theoretical maximum secure speed of the roadway's worst feature (e.g., a curve, bottleneck, hill, etc.). The design speed generally underestimates the maximum secure and safe speed for a roadway and is as a result considered only a very conservative "first guess" at a limit.