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 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)
Use the following to calculate the minimum safe Radius for a curve :
Use the following to calculate the maximum degree of a curve where D is in degrees: