Horizontal curves and superelevation

When a vehicle travels round a horizontal curve, it needs a force towards the centre of the curve to keep turning. That force comes from side friction between tyres and road, helped by tilting the road inward. The tilt is superelevation, e. Without enough of it, a fast vehicle tends to skid outward or, if it has a high centre of gravity, overturn.

Balancing forces gives e + f = V² / (127 R), with V in km/h, R in metres and f the design lateral friction coefficient. Indian practice limits f to about 0.15 and caps superelevation at 7 percent on plain and rolling terrain and in snow-bound areas, 10 percent on hill roads not bound by snow, and about 4 percent on urban stretches with frequent intersections, so a minimum radius follows for each design speed. Designers first check the superelevation that would counteract 75 percent of design speed without friction; if that exceeds the limit, they cap e and check that friction stays within limits at full design speed. If not, speed must be restricted or a flatter curve used.

Superelevation is introduced gradually over the transition length, rotating the pavement about the centre line or an edge. Extra widening is added on sharp curves because rear wheels track inside front wheels and drivers keep away from the edge. Sight distance on the inside of the curve must be kept clear by setting back buildings and cutting slopes, the set-back distance. Consistency matters: a sharp curve after a long straight is a classic crash site.