Extrawidening 2.19


Extra width provide to Road at horizontal curve is called extrawidening.

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.

Extra Widening
Extra Widening

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.

  1. Mechanical widening

  2. Psychological widening


WE= Extra widening

W=Mechanical widening

WP= Psychological widening

(i) Mechanical widening:

Mechanical widening is provided to account off tracking.

Mechanical widening
Mechanical widening

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)





\(W_M=\frac{l^{2}}{2R_2-W_M}≅\frac{l^{2}}{2R}\). where, R= mean radius of curve.


Now for ‘n’ lane Road


(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. 


Where WP in m, R in m, V in KMPH.


WE= \(\frac{nl^{2}}{2R}\)+\(\frac{V}{9.5\sqrt{R}}\).(extra widening formula)


IRC recommended value of extra widening for single and tow lane pavement are given below.

  1. For single lane road Psychological widening =0.

  2. If R> 300 m, then extra widèning is not provide.

  3. If R< 50 m, then extra widèning is provided at inner edge.

  4. If 50<R<300 m, then extra widèning is provided at both the edge.


For Detailed Analysis of Highway Engineering Step By Step.

Highway Engineering

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