In her landmark The Death and Life of Great American Cities, Jane Jacobs defines the success of a neighborhood in terms of people on the streets. A healthy neighborhood, she argues, has people on the streets, doing things and going places, through all hours of the day. This is important, first of all, for safety: as long as there are reasonably responsible "eyes on the street," crime will not be a problem. (She notes that these eyes don't need to be especially responsible: you don't need heroes, you just need people who are awake enough not to let someone get beat up. Despite the horror stories the press likes to blow around -- stories that are very rare and have mostly proved to be be simply untrue -- ordinary people don't stand by while people are getting raped. Criminals know this, and any city dweller knows that the only dangerous street is a quiet one. Note that the grisly murders and rapes happen in Central Park at night, not in "dangerous" Harlem during the day.)
These eyes on the street provide the context for a vibrant street culture, with places to go and things to do. Jacobs calls this "secondary diversity": if you have the right mix of urban fundamentals, the little things that make life interesting pop up. Harvard Square has jugglers and fire-breathers because there are people there to watch them. And any decent neighborhood gets coffee shops, used-book stores, and parks full of people only when there are enough people around to frequent them. (A point Jacobs does not particularly ponder is that there must also be the proper economic conditions. I recently read that to open a home hair-breading salon in Detroit, you need no less than seventy permits: government can squash even healthy neighborhoods.)
Jacobs identifies four material requirements for a vibrant urban neighborhood: density, mixed uses, small blocks, and old buildings. The first two are obvious enough. The first thing a vibrant neighborhood needs is enough people to support all that "secondary diversity"--and indeed, people packed close enough together that you can walk places, stopping to look at things, instead of having to drive.
"Mixed uses" simply means that you have people there during business hours and after-business hours. It's hard to run a restaurant -- or anything else -- in a single-use neighborhood that is empty half the time: you need a lunch crowd and a dinner crowd if you want to make ends meet. And if the sidewalks are mostly empty half the time, they will be taken over by derelicts. Without eyes on the street, bad people move in.
Small blocks are less obvious but equally important. The point is simply that you must be able to walk from point A to point B in various ways. Super-blocks require a person to walk to work -- or the bus stop, or the park, or wherever one goes -- the same way everytime. In so doing, they effectively counteract density. Imagine a dry cleaner's, or a coffee shop: if you can't plan a route from A to B that goes past, you're probably not going to make use of these businesses. Super-blocks effectively build walls through neighborhoods.
But perhaps least obvious is the need for old buildings. Or rather, old buildings are often valued for the wrong reason. Partly claiming Jane Jacobs as an authority, neighborhoods like mine have established "historic districts" to preserve antique beauty. They place all sorts of requirements on owners -- effectively driving out all but the super rich, who can afford wrought-iron fences, manicured gardens, brick walks, and old-fashioned shingles.
But this is exactly opposite to why Jane Jacobs says old buildings are necessary. Old buildings are necessary, she says, because they're cheap. New stuff always carries the price of construction in the mortgage. That's why, for example, malls and strip malls are filled with high-turnover box stores: a poky ma-and-pop place can't afford the rent. Jane Jacobs's greatest fear for her neighborhood was that some do-gooding urban renewer would knock down all the "derelict" old buildings and replace them with brand-new stuff: effectively driving out all but the box stores and fast-food restaurants. Ironically, Jane Jacobs thought a neighborhood could only maintain the beauty of life by having crumbling old buildings for artists, ma-and-pop stores, and all the eccentricity that makes city life great. Those who drive out all but high-priced antiques change the city from a living organism to a museum -- and drive out the middle class in the process.
Old buildings is, I guess, the one place where Jane Jacobs almost understands free-market economics. Frankly, the market is something she never seems to have pondered, libertarian though she was in most respects. Indeed, in standing for old buildings, she partly stands against capitalism, almost begging for the government to protect us from "cataclysmic" development. But even there, Jacobs knew that the real threat to a diverse neighborhood is the government: only eminent domain can clear out blocks at a time.
The real way to stand by Jane Jacobs' old-buildings criteria is not by piling on highfalutin' government museum mandates, but by telling government to leave us alone so that the little guy can have a chance to contribute his necessary two cents to the margins of the neighborhoods. Only the government can take away all the cheap places to do business.