What is design speed and how does it affect highway design?

Design speed is the maximum safe speed that can be maintained over a specific section of highway when conditions are favorable. It directly influences horizontal curve radius, superelevation, stopping sight distance, vertical curve length, and lane width. Higher design speeds require gentler curves, longer sight distances, and wider lanes. Design speed selection considers terrain, traffic volume, functional classification, and economic factors. All geometric elements must be consistent with the chosen design speed.