On the Generators of Diversity: The Simple and Few are Withering

Chapter 7, and the four chapters that follow, are the center and pivot of The Death and Life of Great American Cities. In "The Generators of Diversity," Jacobs makes her most ecological case for the benefits of diversity. In her chapter on block size she will use images that suggest a river delta; in her chapter on building ages she will use images that suggest a forest with old growth and new understory.

In every case, Jacobs argues that the generative power of a jumble vastly outstrips the creative potential of high-theory simplicity and order. Multiplicity and the many will always trump the simple, the neat, and the few.

Cities are Complex, not Simple
A thriving urban ecosystem requires multiplicity and difference. "Diversity is natural to big cities," she says.

A lively city scene is lively largely by virtue of its enormous collection of small elements....Big cities are natural generators of diversity and prolific incubators of new enterprises and ideas of all kinds. Moreover, big cities are the natural economic homes of immense numbers and ranges of small enterprises.

The diversity is economic, with different kinds and shapes and sizes of business, and it is also cultural, with equally multiple forms of recreation, diversion, creativity, and education.

Functional Analysis Over-simplifies and Kills
She starts with a critique of functional analysis.

It is so easy to fall into the trap of contemplating a city's uses one at a time, by categories. Indeed, just this - analysis of cities, use by use - has become a customary planning tactic.

We see this in the Transportation System Plan and its chapters. The Comprehensive Plan and planning code is similarly anatomized.

Instead,

to understand cities, we have to deal outright with combinations or mixtures of uses, not separate uses, as the essential phenomena....A mixture of uses, if it is to be sufficiently complex to sustain city safety, public contact and cross-use, needs an enormous diversity of ingredients.

Economic Diversity is Key
An important part of the diversity, perhaps even the most important part, is economic. One of the reasons cities generate this diversity is the way clusters of small businesses can share the benefits of support services. Large businesses can be closer to self-supporting, and might generate enough business to consume the whole output of a vendor. A vendor needs a larger number of small businesses to sell an equal output. From the other side, small businesses can cluster to generate demand for a given service.

(Happily Jacobs doesn't use jargon. But over at the Oregon Economics Blog, the jargon for this, "agglomeration externalities," is explained without the addition of much more jargon in more detail.)

Size Matters
It's clear that Jacobs is talking about big cities, however. She says

towns and suburbs, for instance, are natural homes for huge supermarkets and for little else in the way of groceries, for standard movie houses or drive-ins and for little else in the way of theater. There are simply not enough people to support further variety, although there may be people (too few of them) who would draw upon it were it there.

Additionally, in towns and suburbs, people are distributed too sparsely, and "when distance inconvenience sets in, the small, the various and the personal wither away."

But size by itself is not enough. She cites the Bronx and Detroit as large failures.

Apparently there is no limit to the numbers of people in a city whose potentiality as city populations can thus be wasted. Consider, for instance, the Bronx, a borough of New York containing some one and a half million people. The Bronx is woefully short of urban vitality, diversity and magnetism....And if the Bronx is a sorry waste of city potentialities, as it is, consider the even more deplorable fact that it is possible for whole cities to exist, whole metropolitan areas, with pitifully limited city diversity and choice....

[Detroit] is ring superimposed upon ring of failed grey belts. Even Detroit's downtown itself cannot produce a respectable amount of diversity. It is dispirited and dull, and almost deserted by seven o'clock of an evening.

It is interesting that even in the 1950s and early 60s, the auto and auto industry had already failed Detroit (though of course other ingredients might factor into its blight as well).

The Four Indispensable Conditions
At the end of the chapter she distills her thesis into four conditions.

To generate exuberant diversity in a city's street's and districts, four conditions are indispensable:

1. The district, and indeed as many of its internal parts as possible, must serve more than one primary function; preferably more than two. These must insure the presence of people who go outdoors on different schedules and are in the place for different purposes, but who are able to use many facilities in common.

2. Most blocks must be short; that is streets and opportunities to turn corners must be frequent.

3. The district must mingle buildings that vary in age and condition, including a good proportion of old ones so that they vary in the economic yield they must produce. This mingling must be fairly close-grained.

4. There must be a sufficiently dense concentration of people, for whatever purposes they may be there. This includes dense concentration in the case of people who are there because of residence.

The necessity for these four conditions is the most important point this book has to make....All four in combination are necessary to generate city diversity; the absence of any one of the four frustrates a district's potential. [emphasis added]

In the next four chapters, we'll drill into these four conditions. What in general we see in Salem is an attempt to simplify and neaten. Salem wants a lively downtown, but it wants only auto traffic, not transit, walking, or bicycling traffic. It wants quiet after 10pm. It wants "aimless" youth who congregate on the transit mall to go elsewhere. It wants a few large institutional employers, not many small entrepreneurial merchants.

Salem wants large chunks of uniformly textured stuff. It is not comfortable with a more kaleidoscopic chaos. Even though Salem might never get the size for real multiplicity, Salem can do more to encourage a creative jumble, a truly generative matrix.