Design Speed - Speed Limit Variance From Design Speed

Speed Limit Variance From Design Speed

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.
• 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 an 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 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".