Most roads are still built this way, your top level of asphalt pavement is only a few inches thick and then will have some manufactured gravel under that to maintain stability. That's where most of your strength comes from, the gravel under the pavement. They're also "crowned", as the OP discussed, to allow drainage of water.

Crushed rock and the science behind it, which is also the science of roadways and pavement, is super neat and one of those "I didn't know it was that complicated" things that people use day to day.