Interesting but somewhat misleading. Each airplane symbol is about 100 miles long on this scale, and the picture shows the three dimensional airspace structure compressed into two dimensions. If the aircraft were shown to the correct scale you wouldn't see them at all; the sky would seem completely clear. The minimum spacing between aircraft in enroute airspace is three miles and it's usually five miles or more, often a lot more. They're also separated in altitude. To quote Truman from Armageddon "...it's a big-ass sky."