**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

**W _{E}= W_{M}+W_{P}**

W_{E}**= **Extra widening

W_{M }=Mechanical widening

W_{P}= 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)

W_{M} = 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 W _{P} in m, R in m, V in KMPH.**

W_{E}= W_{M}+W_{P}

W_{E}= \(\frac{nl^{2}}{2R}\)+\(\frac{V}{9.5\sqrt{R}}\).(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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