Why do people congregate in cities? There are cultural reasons, to be sure, but economic reasoning is at the heart of this decision. Small businesses depend on a vast array of other enterprises for their survival, and thus do best by locating where these other businesses are. Put differently, cities offer benefits to smallness. As Adam Smith said, specialization is limited by the extent of the market.

However, a city is made up of strangers, not all of whom are nice). This makes safety a fundamental concern for city-dwellers. Jacobs argues that safety in cities is not guaranteed by police, but by citizens who have a vested interest in the community. How, then, does she recommend fostering vibrant neighborhoods? There are three key conditions that she identifies for diverse and robust communities.

First, city districts must be mixed-use. That is, it must be usable by people with diverse needs and diverse schedules. If a business lies idle for much of the day it will have difficulty justifying and supporting its continued existence. A central business district that is in use only from 8am to 5pm can only support certain businesses: office work and secondary establishments that cater to those workers (coffee shops and lunch restaurants). These secondary establishments must be able to handle large queues in a relatively short period of time at peak traffic, which means only certain sorts of restaurants (those with a broad appeal and quick service). Other establishments, such as a library or movie theater, cannot survive in such a neighborhood because they would nto have enough off-peak traffic to justify the expense (p. 203). This further compounds the effect of rush hours, since everyone arrives and departs in a short window of time.

The second condition is a varied age and condition of buildings. This allows businesses with varied economic yields to locate near one another. New buildings require highly profitable businesses. A candy shop probably cannot survive on the first floor of a skyscraper. Note that it is businesses, not historical associations, that support the preservation of older buildings. A diversity of uses rather than legislation protects older buildings (and is thus closely related to the previous point). Low rents also allow businesses to experiment with new ideas, and to grow (the ability to move down the street to a larger or nicer location in the future, without losing local customers). This in turn makes a neighborhood more robust: the businesses and the buildings they occupy are of various ages and types, so the odds that they all fail at the same time are greatly reduced.

Jacobs’ final condition for diverse neighborhoods is a dense concentration of people. This supports variety, since homes and business can coexist in a relatively small area. For a longer discussion of the opposite strategy, low-density neighborhoods, see my review of The Power Broker.

I expected to spend much of my time reading this book actively disagreeing with its author’s positions. Instead, I was surprised to find well-reasoned economic arguments. Jacobs is not a supporter of paternal zoning laws, nor of historical preservation for its own sake. The criticism of automobiles is not as strong as you might anticipate from reading secondary sources. This work, especially part two, is well worth reading in the original.