Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Track transition curve
open-in-new
<p>A transition curve (also, spiral easement or, simply, spiral) is a spiral-shaped length of highway or <a href="/facts/Track_(rail_transport)/erncmXfX">railroad track</a> that is used between sections having different profiles and radii, such as between straightaways (<a href="/facts/Tangent/sF0jH3EI">tangents</a>) and curves, or between two different curves. </p> <table><tbody><tr><th>Centripetal force on vehicles on roads without and with a transition curve</th></tr><tr><td> Straight sections of road connected directly by a circular arc. |</td></tr><tr><td> Straight section connected to a circular arc via a Cornu spiral|</td></tr><tr><td>Comparison of a poorly designed road with no transition curve with the sudden application of centripetal force required to move in a circle versus a well designed road in which the centripetal acceleration builds up gradually on a Cornu spiral before being constant on the circular arc. The second animation shows the increasing curvature of the transition curve which is able to connect to a circular arc of progressively smaller radius.</td></tr></tbody></table> <p>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 <a href="/facts/Centripetal_acceleration/UVDmjYvR">lateral (centripetal) acceleration</a> that would otherwise occur without a transition curve. Similarly, on highways, transition curves allow drivers to change steering gradually when entering or exiting curves. </p><p>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 <a href="/facts/Cant_(road%2frail)/VoCP1tAb">bank</a> for the curve. </p>