What term is used to describe the speed used to determine vertical and horizontal alignments of a highway?

The term that describes the speed used to determine vertical and horizontal alignments of a highway is known as design speed. Design speed is a crucial concept in highway engineering as it establishes the basis for the geometric design of roadways, including curves and grades. It represents the maximum safe speed that vehicles can travel on a specific roadway under ideal conditions.

Understanding design speed helps in ensuring that the roadway is constructed to accommodate safe maneuverability, visibility, and comfort for drivers. The design speed influences factors such as the radius of curves and the slope of grades, which are essential for maintaining safe driving conditions. By using a designated design speed, engineers can anticipate potential hazards and optimize roadway features for optimal performance.

In contrast, the other terms mentioned refer to different aspects of roadway and traffic flow. Free-flow speed relates to the speed of vehicles when there are no obstructions, while critical speed involves the dynamics of vehicle stability at certain conditions. Operational speed refers to the actual speed of vehicles on the road under typical traffic conditions, providing insight into how the roadway performs in practice. However, for establishing alignments, design speed specifically serves as the guiding factor.