Highway Superelevation

Superelevation is the banking of roads through turns so that the lateral forces on the vehicle balance out with the centrifugal force during the turn. Proper superelevation will allow a vehicle to safely turn at high speeds and will make riders comfortable.

From the AASHTO Green Book:

Urban areas the maximum superelevation is 4% to 6%.

Ice and snow areas the maximum is 6% to 8%.

Rural areas without ice and snow can have a maximum superelevation of 10% to 12%. It could go higher for most passenger vehicles but slower moving haul trucks and heavy equipment would not be able to travel safely.

Now for some formulas!

$$ e = \tan\phi = \frac{V^2 – fgR}{fV^2 + gR}$$

e is the Superelevation Factor

V is the design speed

R is the curve radius

f is the friction coefficient

g is gravity (33.2 ft/s^2 or 9.81 m/s^2)

$$ \frac{V^2}{gR} = \frac{e+f}{1-ef}$$

$$ e+f \geq \frac{V^2}{gR}$$

Use the following to calculate the minimum safe Radius for a curve \(R_{max}\):

$$ R_{min} = \frac{S^2}{15(0.01e_{max}+f_{design})}$$

Use the following to calculate the maximum degree of a curve \(D_{max}\) where D is in degrees:

$$ D_{max} = \frac{5729.578}{R_{min}}$$