Runways subsurface preparations are much deeper and stronger than even the interstate. There are millions of miles of roadways in the US, and maybe a few thousand miles of runways.
Even Germany’s vaunted Autobahn has an average subsurface preparation of about 28 inches (sand, pebbles, rocks, cement). The US interstate has an average of only 14 inches. Again, it comes down to cost savings.
https://upload.wikimedia.org/wikipedia/commons/c/c2/PavementStratum.JPG
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