Designhastighet

Designhastighet, or design speed in English, is a fundamental concept in road design and traffic engineering. It denotes the speed at which the geometric features of a roadway are planned to accommodate drivers operating the road safely under favorable conditions. It is not the posted speed limit or the typical operating speed; rather, it serves as a design parameter that influences the layout and safety of the road.

The design speed determines several key geometric criteria. It informs horizontal alignment, such as the radius of curves and tangents; vertical alignment, including crest and sag curves and grades; sight distance requirements for stopping and passing; and cross-section elements like lane width, shoulder width, and clear zones. It also guides the use of superelevation (banking) on curves to maintain comfort and control at the target speed.

How design speed is chosen varies by jurisdiction and road function. It is typically selected from standard ranges based on road classification, anticipated traffic, environmental constraints, and safety goals. Different elements along a roadway may use different design speeds—for example, higher speeds on straight rural sections and lower speeds near intersections or settlements. The design speed is used in conjunction with national or regional standards, such as the AASHTO Green Book in the United States or European road design guidelines.

In practice, the design speed aims to ensure adequate sight distance, stopping distance, and lateral stability. It is not a guarantee of actual operating speeds, which can be affected by enforcement, weather, and the driving environment.