Set Back Distance

Set back distance is the clear distance required from center line of the curve to the obstruction on the inner side of the curve to provide adequate sight distances,

Two different cases of calculating setback distance,

Case-1: When length of a horizontal curve is greater than the sight distance (Lc > SD)

• For Single Lane:

Let M is the setback distance and sight distance is measured along the center line, then the setback distance from the center line of road is given,

But we know that R is the radius of a circle then the length of an arc l subtending an angle of α degree at the center is,

l = R.α

length of arc = SD

R.α = SD

by dividing the above equation by 2 ,

 

but α is in degree so, 

• For Multi Lane:

Let 2d is the total width of a lane, then setback distance from the center line of road having only two lane is given,

Now if we have to calculate the setback distance from the center line of inner lane, then it will be given as, For a two lane road, 

and 

Case-2: When the length of horizontal curve is less than the sight distance (Lc < SD)

• For Single Lane:

AC - Setback distance from the center line of the road,

hence,

M = MAB + MBC

so, 

for MBC, consider a triangle PQR 

therefore setback distance from center line of road is, 

but α is in degree so,

• For Two Lane:

 

Setback distance is given as,

M = MAB + MBC

therefore,

and

setback distance from center line of inner lane is, 

and