How Much Density Is Enough? It Depends

Los Angeles has over 7000 people per square mile, yet doesn't have a reputation as a particularly walkable place.  By contrast, I was pretty happy living without a car in Carbondale, IL (a small college town with 2178 people per square mile).  How come?

I would suggest that the bigger a city is, the more dense it needs to be to be walkable and transit-friendly.  For example, suppose that city X has 4000 people and encompasses only 1 square mile, while city Y has 2.1 million people and 300 square miles.  City X is less dense than city Y- it has 4000 people per square mile as opposed to city Y's 7000.  But obviously city X is more walkable: a person of average walking speed can get from any point to any other point in 20 or 30 minutes on foot.  In such a small place, only the elderly and disabled will need public transit. 

By contrast, in a 500-square-mile city, walking outside your neighborhood will be pretty time-consuming.  So you will need New York-level density and transit service to enable most people to function without cars.

So this reality explains Los Angeles: it is dense enough that if it were a small city, it would probably be pretty transit-friendly, but because of its sheer enormousness it isn't.

This reality is relevant to the ongoing debate about high-rises: some commentators seem to be quite focused on the virtues of low-rise urbanism.  In a city of 100,000 people, that might create a lot of lovely, walkable places.  In a city of 3 or 4 million, maybe not.