The healthy city sidewalk does not rely on constant police surveillance to keep it safe, but on an "intricate, almost unconscious, network of voluntary controls and standards among the people themselves, and enforced by the people themselves." Noting that a well-used street is apt to be relatively safe from crime, while a deserted street is apt to be unsafe, Jacobs suggests that a dense volume of human users deters most violent crimes, or at least ensures a critical mass of first responders to mitigate disorderly incidents. The more bustling a street, the more interesting it is for strangers to walk along or watch from inside, creating an ever larger pool of unwitting deputies who might spot early signs of trouble. In other words, healthy sidewalks transform the city's high volume of strangers from a liability to an asset. The self-enforcing mechanism is especially strong when the streets are supervised by their "natural proprietors," individuals who enjoy watching street activity, feel naturally invested in its unspoken codes of conduct, and are confident that others will support their actions if necessary. They form the first line of defense for administering order on the sidewalk, supplemented by police authority when the situation demands it. She further concludes three necessary qualities that a city street needs to maintain safety: 1) a clear demarcation between public and private space; 2) eyes upon the street and sufficient buildings facing streets; 3) continuous eyes on the street to guarantee effective surveillance. Over time, a considerable number of criminological studies have applied the concept of "eyes on the street" in crime prevention. Jacobs contrasts the natural proprietors to the "birds of passage", the transient and uninvested block dwellers who "have not the remotest idea of who takes care of their street, or how." Jacobs warns that, while neighborhoods can absorb a large number of these individuals, "if and when the neighborhood finally becomes them, they will gradually find the streets less secure, they will be vaguely mystified about it, and...they will drift away." Jacobs draws a parallel between empty streets and the deserted corridors, elevators, and stairwells in high-rise public housing projects. These "blind-eyed" spaces, modeled after the upper-class standards for apartment living but lacking the amenities of access control, doormen, elevator men, engaged building management, or related supervisory functions, are ill-equipped to handle strangers, and therefore the presence of strangers becomes "an automatic menace." They are open