# How do you calculate Set back distance on horizontal curve?

Setback distance is defined as  the distance required from the center line of a horizontal curve to an obstruction on the inner side of the curve to provide adequate sight distance at a  horizontal curve.

The Set back distance on horizontal curve depends on following factors:

1. Sight distance ( SSD, ISD and OSD )
2. Radius of the curve R
3. Length of the curve which may be greater or less than S.

Set back distance on horizontal curve applied on the following cases. let us describe below in details.

## Case (a): Le = S

Let the length of curve Le be greater than the sight distance S. The angle subtended by the arc length S at the center be (a-Alpha) . On single lane roads, the sight distance is measured along the center line of the road and the angle subtended at the center.

The distance from the obstruction to the center of the curve is,

Therefore, setback distance in required from the center line is given by:

For multi lane roads, if d is the distance between center line of the road and the center line of the inside lane in meter, the sight distance  is measured along the middle of the inner side lane and the setback distance m is given by:

### Case (b) : Le < S

If the sight distance required is greater than the length of curve Le, then the angle subtended at the center is determined with reference to the length of circular curve Le and setback distance is worked out as given below.

The clearance of obstruction up to  setback distance is important when there is cut slope on  inner side of horizontal curve.

Numerical Example

Q)  A four lane divided highway has a curve 1000 m long and a radius of 550 m , the safe distance of 250 m. calculate the minimum set back distance from the inner edge of a building to ensure safe visibility. The pavement width per lane is 3.5m.

Solution;

Length of curve L > Sight distance S

Distance between the center line of carriageway and the center of inner lane.

Sight distance S = 250 m

Radius of curve R = 550 m

Central angle subtended by  arc of length S

Setback distance from the centre of carriageway

Setback distance  from the edge of the carriageway 