The safety of ice depends on its thickness and other factors such as cracks, inlets, and moving water. Ice begins to be "safe" at around 4 - 6 inches thickness. Do not even walk on ice 3" or less in thickness. However, it is important to note that ice thickness can vary across a body of water and can change.
If you are planning to drive on ice, the ice must be at least 8 inches thick to hold a small pickup truck. It is also recommended to park vehicles at least 50 feet apart or more and move every few hours. Always bring safety gear such as a life jacket, ice pick, cell phone, length of rope, and ice auger. However, even at a 9" - 10" thickness, there may be unforeseen hazards such aa a flowing current underneath that is a ceaselessly weakening the underside of the ice.
If you are unsure if the ice is safe, it is best to avoid it altogether. Remember, there is no such thing as 100% safe ice! Safety should always be your top priority.