San Diego's airport, on the other hand, has the a bustling restaurant district, an interstate with frequent bumper-to-bumper traffic, and a dense residential neighborhood all within a mile off one end of the runway -- and a popular shopping area, an elementary school, and a high school within just over a mile from the other end.

In addition, the terrain rises in both directions (so sharply on one side that planes can't use ILS when landing from that direction).

Move them to where? Cities large enough to merit an airport generally either have development which has expanded around them or physical features not conducive to development (mountains, lakes, etc.).

It's easy to say "just build bigger elsewhere" but unless you go dozens of miles out and add hours to every trip to/from the airport there's no options.

And no, "just fill in every body of water" is not an option. It doesn't work at all in many cases, is hilariously expensive in all cases, and has enormous environmental impact.

>Zoning is one option to direct growth

My magic crystal ball named "the past 50yr of history" says it is unlikely to be the success you envision.

There’s a real convenience to an airport not being 50 minutes away

> The city center requires crossing a bridge.

It actually requires using tunnels or a boat. I used to drive a cab and the I93 + Callahan/Sumner tunnel route was hellish. The Big Dig helped a lot, although sometimes that can get pretty backed up too.

> Look at maps from 1950 and it doesn’t look like a bad location for a small airport.

Generally, airports that are close to major urban centers were developed prior to 1950, including all 3 examples named. Songshan was opened during Taiwan's colonial period as the “Matsuyama Airdrome” serving Japanese military flights (https://www.sups.tp.edu.tw/tsa/en/1-1.htm).

For bigger cities with these old central airports, larger airports were opened later in many cases. I don't think that will ever happen in Boston, although satellite airports in neighboring states like "Manchester-Boston" or TF Greene in Rhode Island try pretty hard.

The Ohio River is a mile wide at Louisville, but that still doesn't wide enough to classify it "large body of water", especially because it is a river that moves relatively quick for its width and then hits falls/rapids just downstream of Louisville.

But also there's a lot of urban and suburban development you'd have to displace to even consider moving the airport near the Ohio River for most miles both up and down stream of Louisville.

I don't think effectively damming (alternatively, rerouting) the Ohio River is a solution to relocating an airport in Louisville. That's a wildly ambitious undertaking compared to most other land reclamation projects.

Yeah, the terrain around Louisville poses a challenge for placing an airport, but they could do like Cincinnati does, and have their airport located across the river. Or place it between Frankfurt and Louisville. Or do like Pittsburg and make the terrain flat enough for an international airport.