When a car is travelling on a banked surface, if it does not slip, this is due to the force of friction acting at the wheels. Suppose the car moves in a horizontal circle of radius r with a speed v. The forces acting on the car are shown in the diagram below.
Resolving vertically gives us
(1)
Resolving horizontally gives us
(2)
gives us
gives us
For the car not to slip,
so
hence