Megan McArdle strikes a skeptical note. Her basic, plausible argument is that existing transportation infrastructure can only sustain so many more residents. I offer some thoughts below:

Unless you plan to fill the city entirely with retirees who don’t need to go to work, there’s actually not that much more room to build up New York–you could put the people there, but they wouldn’t be able to move.

I was surprised that Megan didn’t mention congestion pricing, as it seems like a fairly straightforward way to get more “bang for the buck” infrastructure-wise. Moreover, there is good reason to believe that the number of workers in New York who don’t make traditional commutes is increasing. One could telecommute while still taking advantage of consumption opportunities. Like many other cultural workers, I keep somewhat eccentric hours, and, when the weather is pleasant, I commute on foot. This isn’t viable if you live far from the CBD, but at least some of the bottlenecks are in close proximity to the CBD, e.g., historical preservation districts, resistance to upzoning, etc. Manhattan had a population well in excess of 2 million in the 1920s, concentrated in tenement slums. Modern New Yorkers will expect (slightly) more space and more amenities, but I think it’s fairly clear that Manhattan can sustain a larger population. Megan’s case regarding transit infrastructure is much stronger in the outer boroughs, where, of course, we have much more scope for growth if we can get the complementary infrastructure right.

And even the retirees would require goods and services that choke already very congested entry and exit points. There has been peripatetic talk about switching all deliveries to night, but that would disturb the sleep of low-floor apartment dwellers, and be fantastically expensive, forcing every business to add a night shift.

There are also nontraditional options, including investing in vacuum tubes. This is admittedly an expensive and quixotic option, but I find the idea aesthetically pleasing. And one wonders if we could also use remote-controlled low-flying airships to facilitate deliveries.

At the very least, the current city dwellers are right that adding more people would add a lot more costs to them–crammed train cars, more expensive goods.

This is where Megan implicitly takes into account something like a congestion charge, though I wonder if we could deploy a congestion charge to make infrastructure investments that are welfare-improving.

In New York, much more than in other places, the competition for scarce resources like commuting space is extremely stark. That doesn’t mean it is impossible to add a lot more people to New York. But doing so requires not just changing zoning rules–as far as I know, there’s already quite a lot of real estate in the outer boroughs that could accommodate more people, but it’s not close to transportation, so it’s not economically viable. If you want to add a lot more housing units, you also need to add considerable complementary infrastructure, starting with upgrading the rest of the subway’s Depression-era switching systems (complicated and VERY expensive because unlike other systems, New York’s trains run 24/7).

This is fair. At the same time, these options would become more viable if we built more aggressively in areas that are relatively well-served by transit.

And ultimately, it’s going to mean adding more subway lines, because short of building double-decker streets, there’s no other way for enough people to move.Those lines don’t have to go to the central business district; there’s already been some success developing alternate hubs in Queens and Brooklyn. But they do have to go from residential neighborhoods to somewhere that people work, and they have to add actual extra carrying capacity to the system–line extensions do no good if the trains are already packed to bursting over the high-traffic areas of the route.

The rise of alternate hubs has been particularly striking in the realm of consumption. There are now multiple nightlife destinations in Brooklyn, frequented by taxis. This is a relatively new development, and it parallels the rise of a population that feels little need to leave Brooklyn to meet various needs.

My guess is that easing building restrictions and thus lowering housing prices would create a virtuous circle. Congested infrastructure would spur the rise of social technologies like Weeels that facilitate ride-sharing, and jitney vans embraced by the Afro-Caribbean community might be adopted by people outside of it. A more populous city would also have more tax revenue it could invest in high-quality infrastructure.