Design speed

The design speed of a road is the maximum speed at which a motor vehicle can be operated safely on that road in perfect conditions.

The precise definition, according to the AASHTO Green Book, is "the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of highway." A majority of US states use this definition.


There are a number of factors involved in determining the design speed of a particular road, including its functional classification, its geometric design, the actual operating speed of the road, the maximum speed limit permissible by law, and the traffic volume of the road.

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