Horizontal curves are provided to change the direction or alignment of a road. Horizontal Curves is circular curves or circular arcs. The sharpness of a curve increases as the radius is decreased which makes it risky and dangerous. The main design criterion of a horizontal curve is the provision of an adequate safe stopping sight distance.

The design of the curve is dependent on the intended design speed for the roadway, as well as other factors including drainage and friction. These curves are semicircles as to provide the driver with a constant turning rate with radii determined by the laws of physics surrounding centripetal force.

Types of Horizontal Curve

Simple Curve

A simple arc provided in the road to impose a curve between the two straight lines.

Compound Curve

Combination of two simple curves combined together to curve in the same direction.

Reverse Curve

Combination of two simple curves combined together to curve in the same direction.

Transition or Spiral Curve

A curve that has a varying radius. Its provided with a simple curve and between the simple curves in a compound curve.

Why are Horizontal Curves Needed?
Necessary for gradual change in direction when a direct point of intersection is not feasible. Ex. Highways, Interstates, high-speed roads with a constant flow of traffic

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