On a flat (unbanked) circular road, the centripetal force is provided entirely by friction.

f = $mv^2$/r <= mumg (static friction) => $v_{max}$ = sqrt(mur*g)

If the car exceeds $v_{max}$: It skids outward (not forward). The car slides radially outward, not tangentially.

If the road is wet (mu decreases): $v_{max}$ decreases. Safe speed is lower.

Banking purpose: To provide centripetal force through the normal force component, reducing (or eliminating) the dependence on friction.