Remember that these thicknesses are merely guidelines for new, clear, solid ice.
Many factors other than thickness can cause ice to be unsafe.

Other things to keep in mind when checking ice.
Ice is seldom the same thickness over a single body of water. It can be two feet thick in one place and one inch thick a few yards away due to currents, springs, rotting vegetation or school of rough fish. You need to check the ice at least every 150 feet, especially early in the season or any situation where the thickness varies widely.

White ice, sometimes called "snow ice," is only about one-half as strong as new clear ice so the above thicknesses should be doubled.

Traveling on ice 

Refrain from driving on ice whenever possible.
If you must drive a vehicle, be prepared to leave it in a hurry--keep windows down, unbuckle your seat belt and have a simple emergency plan of action you have discussed with your passengers.

Stay away from alcoholic beverages.
Even "just a couple of beers" are enough to cause a careless error in judgment that could cost you your life. And contrary to common belief, alcohol actually makes you colder rather than warming you up.

Don't "overdrive" your snowmobile's headlight.
At even 30 miles per hour, it can take a much longer distance to stop on ice than your headlight shines. Many fatal snowmobile through-the-ice accidents occur because the machine was traveling too fast for the operator to stop when the headlamp illuminated the hole in the ice.

Wear a life vest under your winter gear.
Or wear one of the new flotation snowmobile suits. And it's a good idea to carry a pair of ice picks that may be homemade or purchased from most well stocked sporting goods stores that cater to winter anglers. It's amazing how difficult it can be to pull yourself back onto the surface of unbroken but wet and slippery ice while wearing a snowmobile suit weighted down with 60 lbs of water. The ice picks really help pulling yourself back onto solid ice. CAUTION: Do NOT wear a flotation device when traveling across the ice in an enclosed vehicle! 

What if you fall in?

 

What should you do? First, try not to panic. This may be easier said than done, unless you have worked out a survival plan in advance. Read through these steps so that you can be prepared.

 Don't remove your winter clothing. Heavy clothes won't drag you down, but instead can trap air to provide warmth and flotation. This is especially true with a snowmobile suit.

1. Turn toward the direction you came. That’s probably the strongest ice. 
2. Place your hands and arms on the unbroken surface. This is where a pair of nails, sharpened screwdrivers or ice picks come in handy in providing the extra traction you need to pull yourself up onto the ice.
3. Kick your feet and dig in your ice picks to work your way back onto the solid ice. If your clothes have trapped a lot of water, you may have to lift yourself partially out of the water on your elbows to let the water drain before starting forward.
4. Lie flat on the ice once you are out and roll away from the hole to keep your weight spread out. This may help prevent you from breaking through again.
5. Get to a warm, dry, sheltered area and re-warm yourself immediately. In moderate to severe cases of cold water hypothermia, you must seek medical attention. Cold blood trapped in your extremities can come rushing back to your heart after you begin to re-warm. The shock of the chilled blood may cause ventricular fibrilation leading to a heart attack and death!   

Source: Minnesota DNR

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