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Copy edits for brevity, clarity, organization, cohesiveness, active voice, simple English for non technical readers, etc.; clarify that spirals can apply to curve–curve not just tangent–curve, and can lower as well as raise to achieve superelevation Tags: Mobile edit Mobile app edit |
Johnjbarton (talk | contribs) Reference edited with ProveIt #proveit |
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[[File:CornuSpiralAnimation.gif|thumb|Animation depicting evolution of a [[Cornu spiral]] with the tangential circle with the same radius of curvature as at its tip, also known as an [[osculating circle]] (click on thumbnail to observe).]]
[[Image:Parabolic transition curve.JPG|thumb|This sign aside a railroad (between [[Ghent]] and [[Bruges]]) indicates the start of the transition curve. A [[parabolic curve]] (''POB'') is used.]]
A '''transition curve''' (also, '''spiral easement''' or, simply, '''spiral''') is a spiral-shaped length of highway or [[track (rail transport)|railroad track]] that is used between sections having different profiles and radii, such as between straightaways ([[tangent]]s) and curves, or between two different curves.<ref name="pwayblog">{{Cite web |last=Constantin |date=03/07/2016 |title=The Clothoid |url=https://pwayblog.com/2016/07/03/the-clothoid/ |access-date=2023-06-07 |website=Pwayblog}}</ref>
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 [[centripetal acceleration|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.
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