What type of curve is used in highway and street vertical alignment?

The correct answer is vertical curve. In the context of highway and street design, vertical curves are used to connect two different grades of a roadway, accommodating changes in elevation efficiently and safely. These curves help to create a smooth transition when a roadway ascends or descends, which is crucial for maintaining visibility, providing comfort to drivers, and ensuring safe vehicle operation.

Vertical curves are classified into two types: crest curves, which are used at the top of a rise, and sag curves, which occur at the bottom of a dip. By incorporating vertical curves into roadway design, engineers can enhance safety and improve driver experience by reducing abrupt changes in slope that could lead to discomfort or loss of control.

While horizontal curves are essential for directional changes on a horizontal plane, and other types like spiral or circular curves may be relevant in specific contexts, they do not pertain to the vertical alignment of roadways. Thus, the application of vertical curves specifically addresses the transition of elevation, making it the appropriate choice for this question.