# What are horizontal curves? types

Last updated on May 14th, 2024 at 03:45 pm

A horizontal curve is a curve in a plan to provide a change in direction to the central line of a road. The minimum radius of a horizontal curve depends on the permissible design speed for the road.

The following types of horizontal curves are used in the alignment of highway:

(a) Simple Curve
(b) Compound curve
(c) Reverse curve
(d) Transition curve

(a) Simple curve: it is a circular curve which consists of a single arc of uniform radius.

This curve is expressed in terms of the degree of the curve, which is the angle subtended at the center by a chord of 30 m length.

This type of curve is suitable for large radius and for slow-moving traffic.

(b) Compound Curve: this is a circular curve which is comprised of a series of two or more simple curves of different radii which turn in the same direction.

This type of curve is used to avoid cutting through hard rocks, heavy cutting or filling, etc. Refer fig.2.4 T1M and MT2 are two adjacent simple curves of radius R1 and R2 respectively and have a common tangent at M.

(c) Reverse Curve: This is a circular curve consisting of two simple curves of the same of different radii which tum in the opposite direction.

These curves are suitable for highways lying in hilly regions. Refer Fig 25 TT and TIT2 are two adjacent simple curves having a common tangent.at T1 and their centre lie on the opposite sides of the curve.

Horizontal Curves in Highways, Extra Widening in Highways || Highway Engineering L-15 ||

## What is a transition curve? What are the objects of providing transition curves? What are their types?

Transition curve: A transition curve is a curve having a radius which decreases from infinity at the tangent to a design of the circular curve. This type of curve is generally introduced on highways between a straight and circular curve to provide ease and gradual change in the direction of a road alignment

Objects of providing transition curve:
(a) To provide a gradual and easy transformation from a straight to circular curve to the straight species
(b) To provide a gradual change of curvature from zero at the tangent point, to that of a circular curve at their junction point.

(c) To enable the gradual introduction of the designed superelevation and extra widening of pavement at the start of the circular curve.
(d) To improve the aesthetic appearance of the road.

Types of transition curves: The following types of transition curves are commonly adopted in horizontal alignment:

(a) Lemniscates
(b) Spiral
(c) Cubic parabola.

The shapes of the three curves are shown in Fig. 2.6.out of the three transition curves, the I.R.C. has recommended the use of a spiral in the horizontal alignment of highways.

### Explain the extra widening of Road on horizontal curves.

On horizontal curves, which are not of very large radi, increased pavement width is provided than the normal width.

This increased width of the pavement is called extra widening.

The extra width of the pavement on the horizontal curve is provided due to the following reasons.

1. To get visibility and a large radius on a curved path the drivers have a tendency to use the outer edge of the curve.
2. While negotiating a horizontal curve, the rear wheels of a vehicle do not follow the same path as that of the front wheels. This is called off tracking, when the inner front wheel takes a path on the inner edge of the curve the inner rear wheel will be off the pavement. Hence requires more of the wheels.
3. At speeds higher than the design speed transverse skidding of the vehicle may occur and the rear wheels of the vehicle may take the outside of the path following by front wheels.
4. The clearance between two vehicles crossing or overtaking on horizontal curves, is kept more than that on a straight road due to psychological effects.
5. Trailer units require even larger extra width at curves.