There is no such thing as 100 percent safe ice. You can't judge the strength of ice just by it's appearance, age, thickness, temperature, or whether or not the ice is covered with snow. Strength is based on all of these factors - plus the depth of the water under the ice, size of the water body, water chemistry and currents, the distribution of the load on the ice, and local climatic conditions.
General ice thickness guidelines from the Minnesota Department of Natural Services (new, clear ice only):
Less than 2 inches - STAY OFF!
4" and thicker - probably safe for walking and ice fishing on foot
5" and thicker - probably safe for ATV or snowmobiling
8-12" and thicker - probably safe for small cars or light pickups
12-15" and thicker - probably safe for medium trucks
See Winter Satey Tips For Walking on Ice & Snow