Which of the following is not an element of a horizontal curve?

In the context of horizontal curves, elements such as long chord, length, and radius are fundamental components that define the curvature of the path. The long chord refers to the straight line connecting the beginning and ending points of the curve, while the length denotes the distance of the curve along its arc. The radius is crucial because it determines the sharpness of the curve—specifically, the distance from the center of the curve to any point on its arc.

In contrast, slope does not pertain to the geometric characteristics of a horizontal curve. Slope typically refers to the inclination of a surface or line in relation to a horizontal plane, which is more relevant to vertical alignment rather than horizontal curvature. Therefore, identifying slope as not being an element of a horizontal curve is accurate, as the other three elements directly influence the curve's geometry and navigation.