Reasons for extra widening

Extra widening is the additional carriageway width given on a horizontal curve beyond the normal width of the straight section.

Reasons

1. Off-tracking: the rear wheels follow a smaller radius path than the front wheels, so a long vehicle needs more width (mechanical widening).
2. Wider swing of the front end: the front overhang sweeps a larger arc outside the track of the front wheels.
3. Driver's difficulty: on curves drivers find it harder to keep to the lane; they tend to drive away from the pavement edges and need extra clearance (psychological widening).
4. Vehicle crossing: a safer clearance between two vehicles passing or overtaking on a curve is needed, as lateral clearances become uneven with speed.
5. Higher speed vehicles need more width for steering.

The extra width is $W_e = \dfrac{n l^2}{2R} + \dfrac{V}{9.5\sqrt{R}}$ and is generally provided when $R < 300$ m; it is introduced gradually along the transition curve.