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.
Geometric features
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:
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 incorporate 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. In reality, the road may have a 45 mph advisory speed on the curve and higher safe operating speeds elsewhere.
The design speed may be higher than legislated speed limit caps, so it would not be legal to sign some roads at their design speeds.
It is based on the capabilities of vehicles and roadways that existed at or before the design speed was determined. Vehicular and roadway technologies generally improve over time. Therefore, as time elapses from when a roadway's original design speed was determined, it is increasingly likely that a design speed will underestimate the maximum safe speed.
Recognizing the limitations on the use of the design speed for speed limit determination, "operating speeds and even posted speed limits can be higher than design speeds without necessarily compromising safety" and "arbitrarily setting lower speed limits at point locations due to a lower inferred design speed is neither effective nor good engineering practice."
Evolution
The concept of design speed is evolving. The definition in the 1994 edition of the AASHTOGreen Book, was "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 used this definition. In 2004, the first sentence was changed to "a selected speed used to determine the various geometric design features of a roadway." This reflects the fact that meeting a minimum design speed is not enough to ensure a safe roadway. Recently, the concept of design consistency has been used instead of minimum design speeds. This attempts to connect driver's expectations about the roadway with the roadway design. It uses driver behavior models to predict vehicle speeds on highway segments, and compares the predicted speed on adjacent segments. Significant reductions in speed from one segment to the next are flagged as locations where drivers may end up driving too fast for road conditions. A major shift in philosophy is also taking place regarding design speed of urban and suburban streets. Highway engineers would measure the prevailing speed on a road, round up to the next multiple of 5 mph, and design the road for that speed, assuming that it would be safe. Recent research and design practices have focused on using the street design to influence drivers to choose a speed appropriate for the neighborhood. This speed has been called the "target speed," and is ideally equal to the speed limit.
Factors
When roads are planned, the selected design speed may be based on or influence several factors, including:
