National's kind of been that way for a long while. I haven't done it in a while, but I used to take the afternoon off and walk the Mt. Vernon trail from downtown to DCA when family would visit. It's a lovely walk, though yes, it's a bit weird that you can walk to an airport.

Not really, most airports are way too big to really be pedestrian accessible. San Diego and DCA are probably among the best, and both are tiny at 650 to 750 acres. Even medium sized airports like ORD are 10 times larger with large airports nearly 60 times larger. You just can't make walking across 7000 acres "easy."

But there are many transit-accessible airports including Chicago and London. Honestly, DCA isn't too bad for that, and this bridge is an improvement. Although the bridge is a typical nova silliness of building a giant ludicrous structure to make up for poor original design.

Yes. It's pretty common actually, especially when you take into account regional and small airports. The real question is, how many international airports are pedestrian-accessible?

DCA isn't international, but it is pretty easy to walk to. But there is a lot of traffic to and from the airport from the Pentagon, as you might imagine, so making that an easier trip is convenient I suppose.