Local authorities and government departments have a remit from society to create lasting value. The experience of the Urban Villages Forum even suggests that some private development companies may be more socially conscious than many local authorities. Society Humans seem to have a need to make enemies. You distrust THEM, the others, people you only read about, people with a different lifestyle, class, religion, language. When you get to know them they are often not so bad after all.
Alienation leads to conflict, graffiti, vandalism, mugging, theft etc.
Urban layouts can either encourage alienation or help people to make contact with each other. Multiplicity of use keeps all parts active. The philosophy that, to regenerate derelict areas, we must provide ideal conditions for commercial development is the most destructive policy for a city's social equilibrium. Good urban layouts can provide the 'passive' social contact that stands alongside the 'active' efforts of clubs, agencies, churches etc. Most informal contact happens when people are walking.