Setback Distance | Formula of Set-back Distance on Horizontal Curves (m’)
Setback Distance | Formula of Set-back Distance on Horizontal Curves (m’)
INTRODUCTION :
Setback Distance:
Setback distance m or the clearance distance is 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.
It depends on:-
- sight distance (OSD, ISD and OSD),
- radius of the curve, and
- length of the curve.
Formula of Set-back Distance on Horizontal Curves (m’):
|
Lc |
= |
Length of the Curve (m) |
|
S |
= |
Sight Distance (m) (either SSD or OSD or ISD) |
|
R |
= |
Radius of horizontal curve (m) |
|
d |
= |
Distance between centerline of road to centerline of inside lane (m) |
|
α |
= |
Angle subtended at the center of horizontal curve
(degrees) |
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
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