Extra Widening-Definition,Requirements and Calculation | Highway Geometric Design

Extra widening is the extra width that is given to the curve of the pavement during its construction. In this article, we have described the different types of extra widening, requirements and its calculation in the below section.


Extra widening
Extra widening

What are the requirements of the extra widening of roads?

You already know that when you drive a vehicle, only the front wheel can be tunred. Then the backwheels follow the front wheel path rather than turning.

But the main problem is, the backwheel does not follow the correct path of the front wheel. The allignment of backwheel will be left to the front wheel if the vehicles turn to the left and vice versa.

This appearance is called off tracking and this phenomenon happens only in short roads. The off-tracking depends on

  1. The radius of horizontal curves
  2. Wheelbase length

The second chance for occurring extra widening is, the driver has a tendency to drive the vehicle through the outer layer of the road, so as to get a maximum vision of roads. So you need to provide an extra width at the outer layer of the roads.

The third chance is, when the vehicle moves higher than design speeds, there will be a chance for slipping the vehicles to the outermost edge while negotiating a curve.


There are two types of extra widening we mentioned below

Types of extra widening of roads

1.Extra widening due to psychological reasons. (Wep)

  While negotiating a horizontal curve, the driver has a tendency to follow the inner of the pavement. This reduces the effective width of the pavement at curves. In order to have a smooth operation at curves, you need to provide an extra widening at the curve of the road.

The IRC recommended a value of extra widening formula for psychological reasons

Wep =  V / 9.5 √r

2.Mechanical Widening (Wem)

 While negotiating a curve, rear wheels have a tendency to follow the inner track. To accommodate this, there will be a widening is provided called mechanical widening have to be provided. This is due to the offtracking of the vehicle.

The off-tracking in a highway is one the main criteria during curve widening design.

The IRC value for the mechanical widening is given by

Wem =  nl² / 2R


  • n=Number of lanes
  • l=Length of the curve
  • R=Radius of curve or curvature


Total extra widening formula is= Wep +Wem

                            = V/ 9.5 √r  + nl² / 2R

Read More

  1. Lane, Terrain, Friction and Camber-Highway Geometric design
  2. SuperElevation-Highway Geometric Design
  3. Grade of Roads-Highway Geometric Design

The Below video shows the curve widening design.

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