It's highly unlikely (due to cost of materials) that there would be more than 12 inches of concrete on a highway. There would maybe be 3 feet of depth including

It's highly unlikely (due to cost of materials) that there would be more than 12 inches of concrete on a highway. There would maybe be 3 feet of depth including base material (dirt, gravel, or other engineered fill types). You would only see that depth of concrete on an airport runway or where the aircraft are parked. I can't seem to find anything definitive about US road depths ATM.