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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 |
add hyphen to spelling of "well-designed" |
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{{Short description|Mathematically-calculated curve in which a straight section changes into a curve}}
[[Image:Easement curve.svg|thumb|The red [[Euler spiral]] is an example of an easement curve between a blue straight line and a circular arc, shown in green.]]
[[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=2016-07-03 |title=The Clothoid |url=https://pwayblog.com/2016/07/03/the-clothoid/ |access-date=2023-06-07 |website=Pwayblog}}</ref>
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! Centripetal force on vehicles on roads without and with a transition curve
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| [[File:Straight-circle.gif|frameless|upright=1.2|center]] Straight sections of road connected directly by a circular arc. |
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| [[File:Straight-cornu.gif|frameless|upright=1.2|center]] Straight section connected to a circular arc via a Cornu spiral|
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| 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.
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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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