Short note: superelevation in hill roads

Superelevation (cant) is the raising of the outer edge of the carriageway above the inner edge at a horizontal curve, so that a component of the vehicle's weight balances the centrifugal force.

Superelevation on hill roads (IRC:SP:48; NRS 2070 follows)

• The superelevation rate is given by

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

with $v$ in km/h and $R$ in m; $f$ = side friction factor, taken as 0.15.

• Maximum value: 10% (1 in 10) on hill roads in non-snow-bound areas; in snow-bound/ice-prone areas not more than 7% (to avoid vehicles sliding at low speed on ice).
• Minimum: equal to the camber, 2.5 to 3% (cross-slope of the road surface).
• On a hairpin, the combined gradient (longitudinal grade plus superelevation) must stay within limits, so the longitudinal grade is reduced on the curve.
• Attainment: the superelevation is introduced gradually over the length of the transition (or, if there is no transition, 2/3 on the straight and 1/3 on the curve), by rotating the pavement about the inner edge (so that the hill-side cut and drain are not affected). The rate of change, i.e. the relative gradient between the raised outer edge and the axis of rotation, should not be steeper than about 1 in 60 in hilly terrain.
• For radii above a limit (when the required $e$ is less than the camber), the road keeps its normal camber section.
• Extra widening is provided on sharp curves along with the superelevation.

For a hairpin with a small radius (14 m or so), the full 10% superelevation is provided, the speed is restricted to 20 km/h, and the curve is on a gentle gradient of 1 in 40 maximum.