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Road curves are regular bends in roads to bring a gradual change of direction. Similar curves are on railways and canals.
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Curves provided in the horizontal plane are known as horizontal curves and are circular or parabolic.
Four types of circular curves are on roads:
Simple curve
A simple curve has the same radius throughout and is a single arc of circle, with two tangents meeting at the intersection (B in this diagram).
Compound curve
A compound curve has two or more simple curves with different radii that bend the same way and are on the same side of a common tangent. In this diagram, MN is the common tangent.
Reverse curve
Also called a serpentine curve, it is the reverse of a compound curve, and two simple curves bent in opposite directions are on opposite sides of the common tangent.
Deviation curve
A deviation curve is simply a combination of two reverse curves. It is used when it is necessary to deviate from given straight path to avoid intervening obstructions such as a building, and place of worship, or river.