Superelevation and conditions for providing it

Superelevation is the raising of the outer edge of the carriageway above the inner edge on a horizontal curve, giving the pavement an inward transverse slope. The component of the vehicle's weight down the slope opposes the centrifugal force, so the vehicle can go round the curve safely and comfortably.

$$e + f = \frac{V^2}{127\,R}$$

Conditions for providing superelevation

1. When the centrifugal ratio is greater than the lateral friction: if $v^2/(gR) > f$ (taken as 0.15 for design), friction alone cannot hold the vehicle, so superelevation is needed.
2. On curves of small to moderate radius at high speed. On very flat curves (large $R$), the required $e$ may be less than the normal camber; then only a cross slope equal to the camber is provided, with no extra superelevation.
3. Design value: $e = V^2/(225R)$ (for 75% of the design speed). If it is less than the camber, a minimum equal to the camber is provided.
4. Upper limit: $e_{max} = 7\%$ in plain and rolling terrain and 10% in hilly terrain (IRC), and 7% in snowy or icy areas, to prevent slow or stopped vehicles from sliding down and to limit overturning.
5. When $e$ comes out more than $e_{max}$, adopt $e_{max}$ and check that $f = V^2/(127R) - e_{max} \le 0.15$. If not, either increase the radius or reduce the speed limit.
6. In urban areas and at junctions, a smaller superelevation (about 4%) is used because of frequent stopping and slow traffic.
7. It must be introduced gradually over the transition length (or tangent) at an allowed rate.