Most ice safety pages reprint a thickness chart: four inches to walk, eight to drive. Michigan does not endorse that chart. Understanding why is more useful than memorizing it.
The Michigan DNR states that there is no reliable inch thickness rule for determining whether ice is safe, and that ice should be tested with a spud, needle bar, or auger. Its stated mantra is that no ice is safe ice.
The familiar chart exists because it is simple, and simplicity is exactly its problem. The DNR position is that ice seldom forms at a uniform rate, so a single number cannot describe a sheet. Three or four inches on a small sheltered pond with no inflow is a completely different material from the same measured thickness over a river with current, or over the open bays of the Great Lakes.
The state is explicit on that last point, and it matters enormously for Michigan specifically: ice covering the bays of the Great Lakes will always be more fragile than the chart implies. Saginaw Bay, Little Bay de Noc, Grand Traverse Bay, and Anchor Bay on Lake St. Clair are all in that category. Four of the six waters tracked on this site are Great Lakes water.
There is a second reason the chart misleads, and it has nothing to do with thickness. Ice quality varies as much as ice depth. New clear ice with a bluish tint is the strongest ice there is. Ice formed from melted and refrozen snow is the weakest, and a foot of that old air filled ice can be less trustworthy than a couple of inches of fresh black ice. A tape measure cannot tell the difference. Your eyes and a spud bar can.
The DNR guidance is to test thickness and quality with a spud bar, needle bar, or auger, and to keep testing as you go rather than once at the edge. Practical version:
The recurring Saginaw Bay incident is not people walking onto obviously bad ice. It is people walking onto genuinely solid ice, fishing for several hours, and finding that an offshore wind opened a crack between them and shore while they were sitting. The ice they tested was fine. The ice they needed to walk back across was not there anymore.
That failure mode is why wind direction belongs on an ice conditions page at all, and why this site shows it. If the wind is pushing ice away from the shore you parked on, that is a reason to reconsider regardless of how much cold has accumulated.
Accumulated cold tells you whether ice should be forming. Satellite cover tells you whether the lake is freezing at a large scale. Neither one measures the ice under your boots, and neither one can. Use this to decide whether the drive is worth it, then make the real decision on the ice with a spud bar in your hand.