Banking a road tilts the normal force inward, providing centripetal force without relying on friction.

Without friction: Nsin(theta) = $mv^2$/r and Ncos(theta) = mg. Dividing: tan(theta) = v^$\frac{2}{rg}$. Only one speed is safe — any deviation causes sliding.

With friction (v > ideal): Friction acts inward and down the bank. $v_{max}$ = sqrt(rg(tan(theta)+mu)/(1-mutan(theta))). If mutan(theta) >= 1, no upper speed limit exists.

With friction (v < ideal): Friction acts outward and up the bank. $v_{min}$ = sqrt(rg(tan(theta)-mu)/(1+mu*tan(theta))). If tan(theta) <= mu, the car can be stationary without sliding.

Flat road (theta = 0): Friction alone provides centripetal force. $v_{max}$ = sqrt(mu*rg).

Design process: Choose theta for the most common speed. Friction handles deviations. Banking + friction gives a wide safe speed range.