Lehrer's NewsHour did a piece on air traffic control, and what ought to be done to keep the planes moving. They talked for several minutes. Just once, briefly, one of 'em mentioned the real problem, lack of airports. Then the flow of vagueness rushed forth, carrying us viewers along into a sea of mush. No discussion of the real problem.
An airport can only do 60 flights an hour. For safety sake, you have to allow the landing aircraft to slow down and clear the runway before the following aircraft can put his wheels down. And you have to let the aircraft taking off get clean off the runway into the air before the following aircraft releases his brakes. Both of these actions take about a minute, so you get a limit of about 60 planes an hour. You can put in dual runways, and use one for takeoff and one for landing, but that's about it.
We have 53 big cities ("standard metropolitan areas") in the country. Some biggies like New York have three airports, most others (Boston, Philadelphia) have only one. Ball park figures, we have 100 airports in the US. ALL the flights have to depart one, and land at another. Once in the air, there is plenty of air to spread 'em out in. The bottleneck is the airports.
What to do? Build more airports, but we all know this is hard. Nobody wants an airport in their back yard, and the things are frightfully expensive. Send more traffic into secondary airports. Actually this works. For instance, Manchester New Hampshire is as easy to drive to as Logan airport for everyone on the north shore. Manchester is very lightly used whereas Logan is jammed. Ten percent of the Logan traffic could go to Manchester and passengers would be happier. Finally, use bigger aircraft, that carry more passengers, and fly them less frequently. The airlines hate this. They want to offer lots of flights so passengers are more likely to fly them rather than a competitor. Frequent departures mean less time to gather passengers, so they operate smaller aircraft, more often. Regulations could be invented to reduce the number of small aircraft flights into bottlenecks like New York.
All the other stuff they talk about (opening military airspace, expensive upgrades to the air traffic control system) are window dressing, or pork for the makers of ATC equipment.