To generate exuberant diversity in a city's streets and districts four conditions are indispensable:1. The district, and indeed as many of its internal parts as possible, must serve more than one primary function; preferably more than two...2. Most blocks must be short; that is, streets and opportunities to turn corners must be frequent.3. The district must mingle buildings that vary in age and condition, including a good proportion of old ones so that they vary in the economic yield they must produce. This mingling must be fairly close-grained.4. There must be a sufficiently dense concentration of people, for whatever purposes they may be there...
Jane Jacobs, The Death and Life of Great American Cities
151, city, planning, urbanism