Here in Minnesota, we always wait at least two weeks after the last report of a car going through the ice... Another true story - we had to pass an ordinance here in the City of Prior Lake to regulate parking on the ice because some of the hot fishing spots were getting too crowded and a bunch of cars and trucks parked too close together were enough to sink the ice even though it was theoretically "thick enough" to drive on. You need both sufficient strength to spread the load over a wide enough area, AND that area has to encompass enough volume of ice to provide the necessary buoyancy. Ice is 91% as dense as water, so each cubic foot of ice only gets you about 5 lbs of buoyancy. Each 5000 lb vehicle is going to need its own 1000 square foot area of ice a foot thick to stay afloat.