Sidewalks
Like … ant colonies, or the cells of a developing embryo, neighborhoods are patterns in time. No one wills them into existence single-handedly; they emerge by a kind of tacit consensus: the artists go here, the investment bankers here, Mexican-Americans here, gays and lesbians here. The great preponderance of city dwellers live by those laws, without any legal authority mandating that compliance. It is the sidewalk — the public space where interactions between neighbors are the most expressive and the most frequent — that helps us create those laws. In the popular democracy of neighborhood formation, we vote with our feet.
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The key here is that sidewalks are important not because they provide an environmentally sound alternative to freeways (though that is also the case) nor because walking is better exercise than driving (though that too is the case) nor because there’s something quaintly old-fashioned about pedestrian-centered towns (that is more a matter of fashion than empirical evidence). In fact, there’s nothing about the physical existence of sidewalks that matters [here]. What matters is that they are the primary conduit for the flow of information between city residents. Neighbors learn from each other because they pass each other — and each other’s stores and dwellings — on the sidewalk. Sidewalks allow relatively high-bandwidth communication between total strangers, and they mix large number of individuals in random configurations. Without the sidewalks, cities would be like ants without a sense of smell, or a colony with too few worker ants. Sidewalks provide both the right kind and the right number of local interactions. They are the gap junctions of city life.