Highway Engineering

The minimum offset clearance Ms required between the centreline of the
highway and the obstruction in question can be estimated in terms of the
required sight distance SD and the radius of curvature of the vehicle’s path R
as follows.
It is assumed that the sight distance lies within the length of the horizontal
curve. The degree of curve is defined as the angle subtended by a 100 m long arc
along the horizontal curve. It measures the sharpness of the curve and can be
related to the radius of the curve as follows:

D=5729.6 ∏R (6.14)

An analysis of the geometry yields the following formula relating the length of
a curve to the degree of curve:

L= 100 ¥ Ds ∏D (6.15)

where Dis the central angle of the curve.
Assuming in this case that the length of the curve is SD, Equation 6.15 can be
written:

SD = 100 ¥ Ds ∏D (6.16)

Substituting Equation 6.14 into the above equation yields:

(6.17)

Since:

Therefore:

(6.18)

Substituting Equation 6.17 into 6.18 the following equation is obtained:

Ms=-R[] 12865 Cos(). ¥SD R (6.19)

Ms=-R[] 12 Cos s()D

Cos

s

Ms

D

2

Ê

Ë

ˆ

̄=-()RR∏

DsSD=¥∏57 296. R

Geometric Alignment and Design 171

Example 6.3

A 2-lane 7.3 m wide single carriageway road has a curve radius of 600 m. The

minimum sight stopping distance required is 160 m.

Calculate the required distance to be kept clear of obstructions in metres.

Solution

Applying Equation 6.19:

Ms =600[1 - Cos(28.65 ¥ 160/600)]

=5.33 m