From a New York Times column by Paul Krugman headlined “City Life, Culture Wars and Conspiracy Theories”:

I have an apartment on New York’s Upper West Side. It’s a very densely populated area — according to census data the area within a one-mile radius of my place has around 100 residents per acre, or more than 60,000 per square mile. This dense (and, to be honest, affluent) population supports a huge variety of businesses: restaurants, groceries, hardware stores, specialty shops of all kinds. Most of what you might want to do or buy is within easy walking distance.