Ice strength depends on a complex mix of factors including its thickness, the daily temperature, whether the ice is covered with snow, the surface size of the river or lake, its depth, the load placed upon the ice, and how fast that load is moving, Thoresen said. Ice bends up and down in long waves that roll away from a car as it drives across the ice. Because of this wave action, drivers who are taking their cars or trucks onto the ice shouldn't follow closely behind other cars or they could cause a breakthrough on otherwise safe ice, Thoresen said, and even thick ice may crack if a car drives above a certain speed on it. People can follow some general guidelines to help them decide whether ice is safe enough to walk or drive upon, Thoresen said. New ice is much stronger than old ice, as is ice formed when the lake freezes instead of ice that forms when snow melts and freezes.
Clear ice is generally stronger than ice with air bubbles in it.
Lake ice is often stronger than river ice, because currents moving under river ice can weaken it. As general guidelines, Thoresen says, people should stay off ice that is 2 inches or less. Ice should be at least 4 inches thick before ice fishing, at least 5 inches thick before people ride a snowmobile or all-terrain vehicle on it, and 8 to 12 inches for a car or small pickup, and 12 inches or more for a medium truck. Also check for local ordinances that may prohibit vehicles from traveling on ice or require vehicles to be equipped with flotation. Check the ice from shore before you go out, and check it frequently as you move around.