Jane Jacobs' concept: safe streets have "eyes on the street" — windows, storefronts, active ground floors that mean people are watching what happens.
The corollary: the modernist superblock — towers set in open green space, with dead zones between buildings — eliminates eyes on the street. The "green space" becomes dangerous because nobody is watching.
This was not a theoretical prediction. Pruitt-Igoe (St. Louis), Cabrini-Green (Chicago), and dozens of other public housing projects confirmed it empirically before the buildings were demolished.
Related: city-as-hypertext