I found this info online, from the Minnesota Dept of Natural Resources. They should know a thing or two about ice safety:
http://www.dnr.state.mn.us/safety/ice/index.html--------------
Ice safety tips
Before venturing out on a frozen lake or pond keep in mind:
There is no such thing as 100 percent safe ice!
Recommended minimum ice thickness
4" of new clear ice is the minimum thickness for travel on foot
5" is minimum for snowmobiles and ATVs
8"- 12" for cars or small trucks
(Remember that these thicknesses are merely guidelines for new, clear, solid ice. Many factors other than thickness can cause ice to be unsafe.)
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When we went motorcycle riding yesterday at 10AM, there was a marked CHP cruiser sitting by Jackson Lake I'm assuming he was there to warn these highly intelligent folks of the dangers of the ice on the lake.
Darwinism at work...
Peace and stay warm and safe!