Is Walkable Urbanism on the Way Out?
So let's start with the big downside that I think is going to plague walkable urbanism for the foreseeable future. It is really expensive. In my neighborhood (which is eminently walkable and well-connected to Manhattan by mass transit, albeit with a 40-70 minute commute), you would have to spend in the $600,000 to $1,000,000 range for a two-bedroom apartment. And many of those apartments carry monthly maintenance fees in the $500 to $1,000 range - and that is not counting property taxes. So New York is increasingly a dense, walkable, livable city that is accessible only to the hyper-affluent. (Other neighborhoods are cheaper, but rents are rapidly increasing almost everywhere in the city, and usually to live somewhere cheaper involves significant tradeoffs in terms of safety, amenities, and commute time.)
Now it's true that living in a sprawling city involves certain financial costs that you don't have to pay in New York. For instance, I think it is truly unnecessary to own a car in New York, whereas in most mid-sized cities in the U.S. it would be a real hardship to live without a car. But bear in mind that a lot of New Yorkers pay $1,344/year in transit costs (that is, 12 30-day unlimited-ride subway cards at $112 each). Owning a car is not that much more expensive than paying $1,344/year. And that's especially true for a family that can get by with one car - in that case, you are saving $2,688/year plus whatever you would have to pay for subway cards for the children in the family. (Living with one car is not workable for everyone, of course, but it will be much easier when cars are self-driving.)
So walkable urbanism is perhaps not an affordable lifestyle for the vast majority of Americans. And although to some degree this is a result of the low supply of urban areas in this country (that is, the supply of housing in walkable cities is unnaturally low because of bad public policy, driving prices up), to some degree I think this is an inevitable function of the way mass transit works. In a walkable city, location is everything. Living 500 feet from the subway station is vastly better than living 2 miles from the subway station, particularly in areas prone to inclement weather. (2 miles may be much more bearable in a climate that is enjoyable year-round.) But if you have a car, 2 extra miles means just a few extra minutes, a very low price to pay. So once an amenity is in place, the effect on land values is spread out tremendously. Instead of a spike of high value around an amenity like a park or a museum or a library, you have a large area of slightly-elevated land prices. It becomes affordable for everyone (or at least, everyone with a car) to have realistic access to the nicest amenities in the city.
Now the sprawled-out cities pay a big cost for their affordability, which is that they use a fair amount of energy for transportation (though less than a lot of people think), the cars pollute the air (though how many small cities have air as dirty as New York's?), and people spend a lot of time in traffic and suffer a lot of injuries/deaths due to car accidents. But if efficient, driverless cars end up dramatically reducing these problems, then really the big remaining problem is that people aren't walking enough and are therefore more prone to obesity and other health problems. That's unfortunate, but there are steps that could be taken to get people to exercise.
So cities may end up being suitable only for the very rich and the very poor: people who can afford to spend $3 million on a house that anywhere else would cost a few hundred thousand, and people who can't afford to own a car (or use driverless cars, if they become available on a non-ownership basis). Sprawling mid-sized cities may be poised for a comeback, and dense walkable urbanism may soon be a niche product without widespread appeal.