Super elevation is one of the most important concepts in highway engineering, especially when designing horizontal curves. Whenever a vehicle moves on a curve, it experiences centrifugal force that pushes it outward. If the road is flat, this outward force can make the vehicle skid or even overturn. To counter this, we raise the outer edge of the pavement relative to the inner edge, creating a safe slope on the curve. This slope is called Super Elevation.
What is Super Elevation?
Super elevation is the transverse slope provided on a horizontal curve in which the outer edge of the pavement is lifted above the inner edge. This helps balance the centrifugal force and allows vehicles to negotiate the curve safely at the design speed. In technical terms, super elevation is denoted by (e) and expressed as e = tan θ, where θ is the angle of banking.
Why is Super Elevation Required?
The main purpose of super elevation is to ensure safety and comfort of road users. On curves, vehicles tend to move outward due to centrifugal force. Super elevation provides the necessary counteracting force. It prevents:
- Skidding – especially during rain when friction reduces.
- Overturning – for heavy vehicles like trucks and buses.
- Excessive wear on tyres and pavement.
- Driver discomfort at high speeds.
A well-designed super elevation helps achieve smooth traffic flow with minimum braking and minimum lateral stress on pavement layers.
Formula for Super Elevation
The general IRC formula used in India for calculating super elevation is:
e = V² / (225R)
where:
e = super elevation rate
V = speed in km/h
R = radius of horizontal curve in metres
IRC allows a maximum super elevation of 7% (0.07) in plain terrain and 10% in hilly terrain.
How Super Elevation is Provided on Site
The construction process involves three steps:
- Attainment of Super Elevation
Before the curve starts (transition curve length), we gradually shift the cross slope from normal camber to full super elevation.
- Pivot Point Selection
Usually, the inner edge or center line is kept as the pivot. The outer edge is raised by the required amount.
- Checking with Level or Total Station
Engineers check the elevation difference between inner and outer edges using levelling instruments to make sure the slope matches the design.
Example Calculation
Suppose the design speed is 80 km/h and curve radius is 300 m.
e = 80² / (225 × 300)
e = 6400 / 67500 = 0.094
Since this exceeds the limit (0.07), we adopt maximum 7% super elevation.
Where Super Elevation Is Used
Super elevation is used on:
Highways
Expressways
Rural roads
Hill roads
Any road having sharp curves
On high-speed corridors like expressways, super elevation is a critical safety component.
Common Mistakes on Site
- Providing super elevation too suddenly instead of using transition length.
- Not matching design elevation with field levels.
- Excess slope causing discomfort for slow vehicles or stopping vehicles.
- Improper drainage on super-elevated curves causing water stagnation.
Conclusion
Super elevation is a scientific way of balancing centrifugal force on curves. A properly designed and executed super elevation improves safety, comfort, vehicle control, and the lifespan of the pavement. For every highway engineer, understanding and checking super elevation on site is a key responsibility to ensure accident-free and smooth traffic flow.
