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.