What is Super Elevation?
Super elevation (also called cant or banking) is the transverse slope deliberately provided on a curved section of road by raising the outer edge relative to the inner edge. Its main purpose is to counteract the centrifugal force acting on a vehicle navigating a horizontal curve, reducing the tendency to overturn or skid outward.
Think of it like a banked running track — the outer lane is raised so runners (or vehicles) don’t feel pushed outward by centrifugal action.
Why is Super Elevation Needed?
When a vehicle travels on a curved path, centrifugal force (P = mv²/R) pushes it outward. On a flat road, this force must be entirely resisted by lateral friction between the tyres and pavement. However, friction alone is often insufficient at higher speeds. Super elevation shares this load by tilting the road itself, so a component of the vehicle’s weight helps counteract the centrifugal force.
Super Elevation Formula
By resolving forces on a banked curve (neglecting small angle approximations for ef term):
e + f = V² / 127R
Where: e = super elevation ratio (rise/width), f = coefficient of lateral friction = 0.15 (IRC), V = design speed in kmph, R = radius of horizontal curve in metres.
In SI units with velocity in m/s: e + f = v²/gR
Equilibrium Super Elevation
When the road is banked such that no lateral friction is mobilised (f = 0), the entire centrifugal force is balanced by the super elevation alone. This condition gives the equilibrium super elevation:
e_eq = V² / gR = (0.278V)² / 9.81R
This results in equal pressure on both inner and outer wheels — the most comfortable condition for passengers.
IRC Maximum Super Elevation Values
| Road Type / Location | Maximum Super Elevation |
|---|---|
| Urban Areas (with mixed traffic) | 4% |
| Plain and Rolling Terrain (rural) | 7% |
| Snow-Bound Areas | 7% |
| Hilly Terrain (not snow-bound) | 10% |
Note: A minimum super elevation of 2–4% (equal to camber) is always maintained for drainage purposes.
Step-by-Step Design of Super Elevation
- Step 1: Calculate e₁ = V²/225R using 75% of design speed (for average conditions).
- Step 2: If e₁ ≤ 7% → design is complete. Provide e = e₁.
- Step 3: If e₁ > 7%, fix e = 7% and calculate f₁ = V²/127R − 0.07. If f₁ ≤ 0.15 → safe, provide e = 7%.
- Step 4: If f₁ > 0.15, calculate allowable speed Vₐ = √(0.22gR). If Vₐ ≥ design speed → provide e = 7%, f = 0.15. Otherwise, restrict speed to Vₐ.
Attainment of Super Elevation
Super elevation cannot be introduced abruptly — it must be gradually transitioned from the normal cambered section. This is done in two stages:
Stage 1: Elimination of Crown
Method A – Outer Edge Rotated About Crown: The outer half is rotated upward until it is level with the crown. Useful when negative super elevation is not too high. Drainage issues may arise.
Method B – Crown Shifted Outward (Diagonal Crown Method): Crown is progressively shifted toward the outer edge. No drainage problem; used when negative super elevation would be excessive.
Stage 2: Rotation of Full Pavement
About Centerline: Equal earthwork cut and fill; no change in centerline profile but drainage may be problematic in cuttings.
About Inner Edge: No drainage issue; centerline profile changes; excess filling required.
Solved Example
Problem: A horizontal curve has R = 240 m, V = 80 kmph, f = 0.15. Find super elevation assuming full lateral friction develops.
Solution: e + f = V²/127R → e + 0.15 = (80)²/(127 × 240) = 6400/30480 = 0.21
∴ e = 0.21 − 0.15 = 0.06 = 6% (within IRC limit of 7% ✔)
Key Summary Points
- Super elevation balances centrifugal force on curves
- IRC formula: e + f = V²/127R
- Maximum SE: 4% (urban), 7% (rural), 10% (hilly)
- Lateral friction coefficient (IRC) = 0.15
- Minimum SE = camber (2–4%) for drainage
- R_min = (0.75V)² / (g × e_max)