Design speed

The design speed is a tool used to determine geometric features of a new road or street during road design. Contrary to the word's implication, the design speed of the road or street is not necessarily its vehicle speed limit or maximum safe speed; that can be higher or lower. Choosing a design speed means finding a balance between several interests which compete for priority, such as high vehicle speeds to allow drivers to travel to their destinations quickly versus low vehicle speeds for the safety of people outside the vehicle (such as pedestrians and cyclists), or quick movement of peak traffic (traffic engineering) versus maximising the economic development potential of the street (urban planning). The design speed chosen for a high-speed roadway 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 design speeds require wider lanes, sweeping curves, wider recovery areas, broader clear zones, steeper curve banking, longer sight distances, and more gentle hill crests and valleys. Roads and streets with lower design speeds require narrower lanes, sharper/tigher curves, smaller or no clear zones, 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: It is only a theoretical or laboratory measurement created before a road is even built. Design speed is not necessarily road's maximum safe speed. As highway design incorporates a significant factor of safety, drivers can travel faster than design speed without difficulty when good weather conditions are present. The highest design speed for a road or segment is the design speed of its least favorable part. For example, given a road segment with a 60 mph design speed except for a curve with a 45 mph design speed, the entire segment would have a 45 mph design speed.