Your chances of getting into Ludington without a reservation are not good. They do not hold any sites for drive-ups. During off-season (while school is still in session) you probably have a fair chance. June through August, not so much. The state parks on the west side of the state, lower peninsula, fill up fast. Once you get away from that area reservations still are necessary but usually more readily available. I would never attempt a drive up in the lower. Might chances in the UP but no reason why I would even do that. Why not just make a reservation and be done with it?