1.NounOf a transport system (such as a road or railway), the assumed vehicle speed used in calculations when designing the system; typically the fastest speed at which it is deemed safe to use the system.
"The road's design speed is only 60mph, as some of the corners are a little sharp."
2.NounOf something that moves (such as a vehicle or motor), the optimal speed as endorsed by its designers; typically in terms of some balance of various factors that are relevant to the object's purpose, such as longevity, efficiency and velocity.
"The vessel's design speed is 20 knots, though we can push her up to 25 if necessary."