AASHTO a Policy on Geometric Design of Highway and Streets 2011 6th Ed (Green Book)
The design speed is a tool used to determine geometric features of a new road during road design. Contrary to the word's implication, a road's design speed is not necessarily its maximum safe speed; that can be higher or lower. The design speed chosen for a highway is a major factor in choosing superelevation rates and radii of curves, sight distance , and the lengths of crest and sag vertical curves. Roads with higher travel speeds require sweeping curves, steeper curve banking, longer sight distances, and more gentle hill crests and valleys. Lower speed roads can have sharper curves, less banking, less sight distance, and sharper hill crests and valleys. While a road's design speed is sometimes used to determine an initial speed limit, it is an imperfect measure of the maximum speed at which a motor vehicle can be operated for reasons including:.
Required level? Or agreed level? Pitching elements of the Green Book as a requirement allows engineers to deflect questions about their decisions. Not intended to be prescriptive. A presentation done by the Kentucky Transportation Cabinet correctly notes that the Green Book is not a standard and should not be referred to as a standard.