Extra Widening Of Road Types And Formula

This Article Covers What Is “Extra Widening” Types And Formula Of ExtraWidening One By One.

Extra Widening

Extra widening refers to the additional width of carriageway that required on a curved section of a road over and above that required on a straight alignment.

The vehicle has occupies more width that it occupies on straight portion of the road. To compensate this, the carriageway width increased on the entire curved portion of the road, which called extra widening.

This widening done due to two reasons:-

  • Mechanical Widening
  • Psychological Widening

Mechanical Widening (Wm)

The reasons for the mechanical widening are When a vehicle negotiates a horizontal curve, the rear wheels follow a path of shorter radius than the front wheels.

Wm = nl²/2R 

Psychological Widening (Wps)

Widening of pavements has to be done for some psychological reasons also. There is a tendency for the drivers to drive close to the edges of the pavement on curves.

Some extra space is to be provided for more clearance for the crossing and overtaking operations on curves. IRC proposed an empirical relation for the psychological.

Wps = V/9.5√R

Extra Widening Formula

The total extrawidening (We) required on a horizontal curve Calculate By,

We (in m) = Mechanical widening(Wm) + Psychological widening(Wps)

= nl²/2R + V/9.5√R


  • n= number of the traffic lane,
  • l= length of the wheelbase of the longer vehicle in m (generally taken as 6 m),
  • V= design speed of the vehicle in Km/h and
  • R= radius of the horizontal curve in meter

Why Extra Widening Used

In order to provide a smooth drive, when vehicle take turn at a curve its back portion slips due to which vehicle goes off track so that’s why extrawidening is used.

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