ExtraWidening:
Extra width provide to Road at horizontal curve is called extrawidening.
Note: This topic is part of the second chapter of Highway Engineering. I suggest reading this topic in the context of the complete chapter: Geometric Design of Highway. If you want to read the entire Highway Engineering, click here: Highway Engineering.
It is provided for following purposes:
To avoid off tracking due to rigidity of wheel base.
To encounter psychological tendency of driver.
To increase visibility at curve.
To account lateral skidding.
Off tracking:
Path travelled by front axle and path travelled by rear axle it is not same on horizontal curve this phenomena is called off tracking.
Extrawidening is split up into two parts.
Mechanical widening
Psychological widening
WE= WM+WP
WE= Extra widening
WM =Mechanical widening
WP= Psychological widening
(i) Mechanical widening:
Mechanical widening is provided to account off tracking.
R₁ = Radius of path traversed by the outer rear wheel (in m) wheel (in m)
R2 = Radius of path traversed by the outer front wheel (in m)
l = Length of wheel base (in m)
WM = Mechanical widening (in m)
\(R_2^{2}=R_1^{2}+l^{2}\).
\(R_2^{2}=(R_2-W_M)^{2}+l^{2}\).
\(R_2^{2}=R_2^{2}-2R_2W_M+W_M^{2}+l^{2}\).
\(2R_2W_M-W_M^{2}=l^{2}\).
\(W_M=\frac{l^{2}}{2R_2-W_M}≅\frac{l^{2}}{2R}\). where, R= mean radius of curve.
\(W_M=\frac{l^{2}}{2R}\).
Now for ‘n’ lane Road
\(W_M=\frac{nl^{2}}{2R}\).
(ii) Psychological widening:
There is a psychological tendency of driver to drive the vehicle close to the edge of pavement and maintain more side gap between the vehicle moving parallely.
\(W_P=\frac{V}{9.5\sqrt{R}}\).
Where WP in m, R in m, V in KMPH.
WE= WM+WP
WE= \(\frac{nl^{2}}{2R}\)+\(\frac{V}{9.5\sqrt{R}}\).(extra widening formula)
Note
IRC recommended value of extra widening for single and tow lane pavement are given below.
For single lane road Psychological widening =0.
If R> 300 m, then extra widèning is not provide.
If R< 50 m, then extra widèning is provided at inner edge.
If 50<R<300 m, then extra widèning is provided at both the edge.
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