"... if a city's streets are safe from barbarism and fear, the city is thereby tolerably safe from barbarism and fear. When people say that a city, or a part of it, is dangerous or is a jungle, what they mean primarily is that they do not feel safe on the sidewalks.
But sidewalks and those who use them are not passive beneficiaries of safety or helpless victims of danger. Sidewalks, their bordering uses, and their users, are active participants in the drama of civilization versus barbarism in cities. To keep the city safe is a fundamental task of a city's streets and its sidewalks.
This task is totally unlike any service that sidewalks and streets in little towns or true suburbs are called upon to do. Great cities are not like towns, only larger. They are not like suburbs, only denser. They differ from towns and suburbs in basic ways, and one of these is that cities are, by definition, full of strangers ...
The bedrock attribute of a successful city district is that a person must feel personally safe and secure on the street among all these strangers. ... Today barbarism has taken over many city streets, or people fear it has, which comes to much the same thing in the end. ... It does not take many incidents of violence on a city street, or in a city district, to make people fear the streets. And as they fear them, they use them less, which makes the streets still more unsafe.
... the public peace -- the sidewalk and street peace -- of cities is not kept primarily by the police ... 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 -- older public housing projects and streets with very high population turnover are often conspicuous examples -- the keeping of public sidewalk law and order is left almost entirely to the police and special guards. Such places are jungles. No amount of police can enforce civilization where the normal, casual enforcement of it has broken down."
-- Jane Jacobs The Death and Life of Great American Cities
(check out the quotes and links on Gwenda Bond's obit posting on Jacobs.)