What describes a compound curve in highway design?

A compound curve in highway design is defined as a curve consisting of two connected curves, typically with differing radii. This configuration allows for a smoother transition for vehicles moving along a highway, making it safer and more efficient, especially in areas where the geometry of the road must adapt to the terrain or other constraints.

In contrast to a single curve, which is a simple arc, and a series of straight lines, which do not provide any curvature, a compound curve offers a more gradual path change. While the option mentioning three connected curves describes a more complex geometry, it does not fit the standard definition of a compound curve, as that typically involves just two curves. Thus, the correct choice accurately captures the essential characteristics of a compound curve used in highway design, emphasizing its role in enhancing the performance and safety of roadways.