What does "vertical alignment" refer to in highway design?

In highway design, "vertical alignment" specifically pertains to the upward and downward curves of a roadway along its profile. This aspect of highway design is crucial because it directly influences the safety and comfort of vehicles traveling on the road. Vertical alignment includes elements such as crests (upward curves) and valleys (downward curves), which help to manage sight distance, control speed, facilitate drainage, and ensure proper vehicle handling. A well-designed vertical alignment can prevent issues such as excessive grade, which may impact a vehicle’s performance, or insufficient sight distance, which could lead to accidents.

In contrast, the other options touch on different aspects of roadway design. The layout of the roadway as it curves left or right refers to horizontal alignment, the location of bridges pertains to structural design considerations, and the distance between lanes on a highway relates to lane width specifications. Each of these elements contributes to overall roadway functionality, but they are distinct from the concept of vertical alignment, which is focused solely on the elevation changes along the roadway profile.