Speed limit

A speed limit is the maximum speed allowed by law for vehicles on a road. (Also, an axiom of Einstein's relativity theories states that the speed limit of the Universe is the speed of light in a vacuum, i.e., 299,792,458 metres per second.)

Design speed

Speed limits are only peripherally related to the design speed of the road.

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In the United States, the design speed is "a selected speed used to determine 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 "maximum safe speed that can be maintained over a specific section of highway when conditions are so favorable that the design features of the highway govern."

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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 Current U.S. standards for design speed derive from outdated, less-capable automotive technology.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 (e.g., a curve, bottleneck, hill, etc.). The design speed usually underestimates the maximum safe speed for a roadway and is therefore considered only a very conservative "first guess" at a limit.

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