A city or a part of a city works well, according to Jacobs, when there is presence of people - inhabitants, workers, visitors, tourists - during the whole day and evening. She maintains that, in order to obtain such an effect, cities and parts of cities (districts, neighborhoods, etc.) must be diverse - diverse in usages, that is. It is very easy to contemplate that in an area, where people both live and work (not necessarily the same people, of course), people are present, in the streets, the whole time: people going to and from work, parents taking their children out for a stroll, people living there, enjoying their walk after work, people going out to dine or enjoy themselves in some other fashion.
This means that the city and its streets in particular are monitored by the city's dwellers at all hours - and also that people get many more chances to meet each other, to bump into each other, to discover people with similar interests or desires. Jacobs claims that there are four generators of such diversity: mixed primary uses, small blocks, old buildings, and concentration. Let me just write a few words for the first three generators of diversity - I will deal with the need for concentration on my next post on the book.
Mixed primary uses means that a part of a city cannot be solely a business district or an art center or residential. It needs to mix up at least two primary usages, in order for people to be frequenting it all day. This, in turn, means that many secondary uses can be developed, since there are many businesses that can thrive on a clientele present not only for a couple of hours a day (as is the case in business districts, for example), but can count on customers coming in any time of the day. Restaurants and cafes are a typical example of businesses that cannot be sustainable if they work for only two to three hours a day - their overhead, then, is too much for them to handle, they have to raise prices, which means they lose clients and, inevitably, they are forced to close. Conversely, if they can be sustained and be open all through the day, they can support new businesses or new residences, since the workers or the dwellers can count on these businesses for much of the coverage of their needs. They can also attract outsiders, which is of course very good for the local economy, leading to the opportunity for new businesses, and so on.
Small blocks are another generator for diversity, in that they allow many small streets to form and the people living in them get more opportunities to mingle and to follow different paths everyday to their work or to their bus stop. These small streets are also places for small businesses to appear and to assist in mixed primary or secondary uses, leading to the results just mentioned. If, on the other hand, blocks are large, people are reduced to taking one particular route for their everyday walks or strolls or to go to their work. Moreover, it is much more difficult for small businesses to develop, since the distances between a point on the opposite site of the block and the small business is so big, as to discourage potential customers from making the trip (as it often is) from their houses to that business, be it a restaurant, a barber shop, a small gallery, or whatever. Small neighborhood businesses in residential areas bring, of course, the mixed use Jacobs considers important to generate diversity. Jacobs attributes the development of Greenwich Village to the East (to what today is known as the East Village - remember, this book was published in 1961) and not to Chelsea, which was up to some point comparable to Greenwich Village in terms of its population's incomes, to the difference between the large blocks of Chelsea and the small ones in Eastern Village.
Old buildings or, rather, a mix of old and new buildings, seems at first glance an unlikely generator of positive diversity. However, one can understand that cheap buildings, cheap land in general, is required for many an upstart company, which cannot afford to pay expensive rents or buy a studio, an apartment or a store at a high price. Newer and more expensive buildings can be used by established businesses and the osmosis between small and big, upstart and established, proves mutually beneficial in almost all cases.