Types of Curves


For the comfort and ease of drivers, mainly two types of curves are provided in the road, curves play an important role in the geometric design of road and railway alignment.

Therefore, it must be properly studied and designed, so that safety, comfort and convenience can be provided or accidents can be reduced while driving vehicles.

Curves are regular bends provided in the lines of communication like roads, railways and canals etc. to bring about gradual change of direction.


They enable the vehicle to pass from one path to another when the two paths meet at an angle.

They are also used within the vertical plane at all changes of grade to avoid the abrupt change of grade on the apex.

Necessity of different curves:

  • Excessive cutting or filling can be prevented by providing the change in alignment by the curve.
  • The obstruction like natural or artificial which comes in the way of straight alignment can be made easier in providing the by-pass with the help of curves.
  • Within the straight route, gradients are made extra comfortable and easy by offering diversions with the help of curves.
  • In a straight route, if costly land comes in the way, then it can be avoided by providing diversions with the help of curves.
  • Track or road is made the stable and safe side of the hill by changing the alignment.

Different types of curves:

Horizontal Curves:

The road or track is called horizontal curves to achieve a gradual change in the directions provided at a bend or turn.

In railway tracks, the minimum radius of a horizontal curve for broad gauge should be 175 meters.


In the case of horizontal as well as vertical, road curves should ensure safety, comfort and convenience for traffic, they are further classified as follows:

1.Simple Curve:

A curve in which there is an arc of a circle in which two straight tangents join and bring a deflection of the road through an angle is called a simple curve.

The simple curve is normally represented by the length of its radius or the degree of the curve.

This type of curve is provided at every change in the alignment of a road or railway track in a plain and hilly area.

2.Compound Curve:

A curve with a series of two more simple curves of different radii curves in the same direction is called a compound curve.

In the compound curve, the two adjoining curves may have a common tangent and the centers of the two adjoining curves might be positioned on the identical side of the curve.

In order to avoid erosion through hard rocks, heavy cutting or filling in the alignment of road or track, mixed curves are provided.

3.Reverse curve:

The curve which consists of two simple curves, which are equal or different rad in the opposite direction, is called reverse curve.

The two centers of the curves are on opposite sides of a common tangent.

Reverse turning is required on mountainous routes, where frequent changes in the direction of travel and the alignment of railway tracks in station yards and in hilly areas.


4.Transition curve:

A curve in which the radius gradually varies from infinity to a finite value that is equal to a circular curve and vice versa is called the transition curve.

These curves are commonly used in railway tracks between circular curves and tangents to provide an easy and gradual change in direction.

This provides a gradual change in the radius of curvature.

In addition, provide smooth entry of the vehicle from the straight part to the curved part to allow the gradual application of overgrowth and additional widening on horizontal curves.

Vertical curves:

The curves provided in the alignment of the road or track at the time of gradient change are called vertical curves.

This provides adequate visibility and security to the traffic.

This gives a gradual change in grade or slope and gives sufficient comfort to the passengers.

1.Summit Curve:

Curves with convex surface upwards are called summit curve.

The curves which are having convex surface on upward side is termed as summit curve.

2.Valley Curve:

Curves with convex surface at the bottom are called valley curve.



Functions of types of Curves in Road and Track Alignment:

  • A gradual change in gradient in alignment can be done by providing curves.
  • A gradual change in direction or orientation in the alignment can be made by providing curves.
  • Curves have been provided to provide comfort to the passengers.
  • Curves have been provided for easy diversion in terms of road and track.


A curve is a continuous and smooth flowing line without any sharp turn, to recognize the curve is that it bends and changes its direction at least once.


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