Banked Turn
v = [g r {sin(q)+m cos(q)} / {cos(q) - m sin(q)} ]1/2
Note that as m sin(q) approaches cos(q), the denominator approaches zero, so the maximum speed approaches infinity!
What force really supports such large speeds (and so large accelerations)? As the angle increases, the contact force begins to act more and more to cause the acceleration. And as the contact force increases, so does friction.
Actually, there is a limit on the maximum speed because there is a limit to the contact force.