ENGINEERING – TRANPORTATION ENGINEERING CALCULATOR
Superelevation Runoff
A precise tool.
What is the Superelevation Runoff & How does it work?
Superelevation runoff length is the distance required for a vehicle to safely transition from a curved, superelevated section of roadway back to a level, straight alignment. This distance ensures that the vehicle’s lateral acceleration is gradually reduced, preventing loss of control.
The required length depends on the design speed (V), the superelevation rate (e), and the side friction factor (f). Higher speeds or lower combined e+f values increase the needed runoff distance.
L = \frac{V^{2}}{127\,(e+f)}
L = runoff length (ft)
V = design speed (mph)
e = superelevation rate (ft/ft)
f = side friction factor (unitless)
V = design speed (mph)
e = superelevation rate (ft/ft)
f = side friction factor (unitless)
Engineers use this formula to design safe transition zones on highways, ensuring that drivers experience a smooth change in lateral forces as they exit a curve.
Parameters
Result
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Frequently Asked Questions
What is superelevation runoff?
Superelevation runoff is the distance needed for a vehicle to safely transition from a curved, superelevated section of roadway back to a level alignment.
How does design speed affect superelevation runoff length?
Higher design speeds require longer superelevation runoff lengths to ensure safe transition and prevent loss of control.
What is the formula for calculating superelevation runoff length?
The formula is L = V^2 / (12 * (e + f)), where V is design speed, e is superelevation rate, and f is side friction factor.
How does the superelevation rate affect runoff distance?
A higher superelevation rate (e) can reduce the required runoff length, but it must be balanced with the side friction factor (f).
Why is side friction important in this calculation?
Side friction (f) affects how quickly a vehicle can transition from a curved alignment. Lower f values require longer runoff distances.
Can you provide an example of calculating superelevation runoff length?
For a design speed of 60 mph, a superelevation rate of 5%, and a side friction factor of 0.12, the runoff length L would be approximately 83 feet.
What are the units for the variables in the formula?
Design speed (V) is typically in miles per hour (mph), superelevation rate (e) is a percentage, and side friction factor (f) is dimensionless.
Results are for informational purposes only and do not constitute professional advice.