Track transition curve
A transition curve (also, spiral easement or, simply, spiral) is a spiral-shaped length of highway or railroad track that is used between sections having different profiles and radii, such as between straightaways (tangents) and curves, or between two different curves.[1]
In the horizontal plane, the radius of a transition curve varies continually over its length between the disparate radii of the sections that it joins—for example, from infinite radius at a tangent to the nominal radius of a smooth curve. The resulting spiral provides a gradual, eased transition, preventing undesirable sudden, abrupt changes in lateral (centripetal) acceleration that would otherwise occur without a transition curve. Similarly, on highways, transition curves allow drivers to change steering gradually when entering or exiting curves.
Transition curves also serve as a transition in the vertical plane, whereby the elevation of the inside or outside of the curve is lowered or raised to reach the nominal amount of bank for the curve.