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.