The first thing to understand is that the public peace—the sidewalk and street peace—of cities is not kept primarily by the police, necessary as police are. It is kept primarily by an intricate, almost unconscious, network of voluntary controls and standards among the people themselves, and enforced by the people themselves. In some city areas—olde

refusing or subverting an unspoken custom, revealed its often-fragile contours. For a moment, the custom is shown to be not the horizon of possibility, but rather a tiny island in a sea of unexamined alternatives.

But desire lines may also be evidence of something more than pure practicality. The casual disobedience of a desire path as an alternative to the formally prescribed walkway is remarkable simply as a human choice, willfully out of step with the way things are.