tl;dr: expanding cities underground is a good thing
I was talking to someone the other day who moved from NYC to the Raleigh/Durham area. Because I used to live in Manhattan, we quickly moved into that shorthand of describing places by naming the nearest subway station. If I'm telling someone that I used to live on Bleeker & Thompson, they might know where that is. If I tell them I lived 2 blocks east of the West 4th Street Station, they know exactly where that is. In fact, when I asked the New York transplant what they missed most about the city, they instantly said the mass transit system. New York is an amazing city to live in without a car . Raleigh, NC is not a place that can make a similar claim. There are several reasons for this, but the one I want to discuss is population density.
The city of New York is the most densely populated area in the US. At almost 28,000 people per square mile, it is well ahead of the second most crowded city, San Francisco, at just under 18,000 residents. If you include the New Jersey cities of Jersey City and Patterson City, the NY Metropolitan area accounts for 3 of the top 4 densest communities in the country. Raleigh's population density? A measly 3,337 people per sq. mile. Currently, Raleigh is the 41st largest city in the US and the Raleigh-Cary MSA (Metropolitan Statistical Area) is also ranked 41st at 1,448,411 people as of 2021. The New York-Newark-Jersey City, NY-NJ-CT-PA MSA is the largest in the nation, with 19,768,458 residents. This MSA requires amazing public transportation systems because the infrastructure simply cannot handle the volume of vehicles that would exist if everyone had a car. I can tell you from first-hand experience that the very idea of owning a car when living in Manhattan seemed far more complicated and annoying than simply getting around via subway and taxis.
As vibrant as New York is, the city is actually too crowded. Raleigh, on the other hand, isn't crowded enough. This is starting to remind me of the children's fable about Goldilocks and the three bears. However, there is actually an ideal population density to strive toward - the perfect level of crowdedness that many in urban planning know as "the 15-Minute City." This term describes a city where all residents have access to essential needs within a 15-minute walk or bike ride. This term was coined by Carlos Moreno of Pantheon Sorbonne University in Paris, France. His four key characteristics of a 15-minute city are proximity, diversity, density, and ubiquity. In order to achieve this, the population density would need to be about 12,900 people per sq. mile. Chicago, IL comes in at 11,942 people, although the land area is staggeringly high at 227.7 sq. miles. But, the good news is that you now have an ideal population density goal, and you have metrics of how that ideally looks with the 15-Minute City model! As my Greenheart co-founder would love to say at this point, "Let's gooooooooo!!!!!!"
Sadly, almost all urban areas in the US cannot easily satisfy the four characteristics at this moment and the path toward achieving this is actually fairly daunting. The reason for this is because municipal boundaries are fairly established now. There are only 10 municipalities in the US that need to get people out of their boundaries because their population densities are too high, but most will never want to do this because of tax revenue loss. All the other cities need to jack up their population densities, and by quite a bit. Density is a term that gives many city planners heartburn. The reason is that they associate density with higher levels of traffic, crime, and, let's be honest, more people of color. Therefore, any expansion of population is usually out, and not up. This is why densities in the US haven't increased much over the years and why becoming denser is staggeringly difficult. The stark reality is that we must become denser, and the answer is developing upward, unless we do something similar to what Seoul, Montreal, Helsinki, and a few other cities have done, which is to expand underground.
For the longest time, I have been saying that suburbs need to expand vertically. We need to have multi-level buildings instead of pushing municipal boundaries further and further outward. The suburban housing development is not only visually unappealing, it also is a titanic waste of resources and the absolute dumbest way to live when looking through the lens of sustainability. However, it is a little unfair to tell someone that they cannot have a key piece of the "American Dream", which is the white picket fence, yard, and McMansion in a housing development that has a name directly ripped off from a John Hughes movie. Every community in the US has a Laurel Hill Estates, a Quail Crest, a Fox Meadows, etc. etc. etc. I consider places like this to be similar to tattoos. In the quest to be unique, you end up being just like a lot of other people. Ironic, isn't it? Anyway, I'm not here to criticize people's housing choices, I'm here to discuss how we can have our cake and eat it, too when it comes to population density and providing neat services like public transportation to everyone who wants it. And that answer is to start digging.
What if, under the cookie-cutter housing developments, there was a robust commercial and service world? This is not an uncommon thing; I vividly remember being shocked at how much shopping, dining, and other services existed in between two subway stations in Seoul, S. Korea. It was an entire mall underground. Montreal's RÉSO, more popularly called Underground City, is used by half a million people a day and is a wonderful place to be in the wintertime. The issue with how US cities are designed is that residential, commercial and governmental needs all compete for the same space. Montreal and Seoul have utilized subterranean space for commercial needs because a store doesn't really need direct sunlight or a view of the mountains. It actually works very well because it is easy for commuters to shop for their needs on their way home instead of making a trip to a grocery store with its huge parking lots.
Pictured below is the famous Itäkeskus Swimming Hall, which was created in 1993. Incredibly, this amazing facility can also be an emergency shelter for over 1,000 people. Itäkeskus is just one of Helsinki's underground assets, ranging from a hockey rink, church, parking lots, and, of course, shopping malls. Helsinki started this downward expansion in 1960 for two purposes. The first was to escape the damn cold because it gets a wee bit frigid there in the wintertime and the second was to become more energy efficient. Finland has taken underground expansion to the point that the underground city could, in an emergency, house 900,000 people in 500 various shelters. For the sake of comparison, the current population of Helsinki is 650,000. Downward expansion is not a theoretical concept, it is occurring in cities right now, just not that many in the US.
The frustrating thing is that many US cities have absolutely incredible underground infrastructure already. Many cities have abandoned underground railroad tunnels. Indianapolis, IN has unused catacombs under its City Market district. Bootleggers developed quite intricate subterranean infrastructure in cities like Los Angeles and the twin cities of Minneapolis and St. Paul. Hell, even the Mormons got into the underground action with their Temple Square tunnels. Burrowing into the ground isn't possible in some locations. Go look for a basement in South Florida. You won't find one, and for damn good reason. Many areas in the southeastern parts of the US can't support underground expansion at the moment, because we don't have the technology to address clay soil. Maybe we just need to try a little harder, though. Or, go digging a wee bit deeper into the bedrock. Yes, it's more expensive, but the opportunity cost exceeds the expense and risk. Cities of the future will have underground components in them. It is up to us as Americans to determine if we want to stay stuck in the habits that make a bad problem worse or move onward and downward.