One of the most stunning achievements of 9-11 was how fast the air traffic control system put everyone on the ground. The "before" picture is around 0930 or so, with about 10,000 planes in the air nationwide. 60 minutes later, there were about 150 (the "after" picture), and most of them were circling runways waiting for other planes to clear so they could land.
In one hour, they put over nine *thousand* planes on the ground.