The three primary Centripetal Forces on the urban form are; Economies of Scale, Economies of Agglomeration and Positive Locational Externalities.  I’m mostly interested in the economy of agglomeration as it addresses the various linkages in the production process.  Upstream and downstream ‘vertical’ linkages are well understood.  Horizontal linkages ‘synergies’ provide for shared knowledge between firms & disciplines and skilled work force development.  What is less understood is how these linkages cultivate demand for a more diverse & dynamic set of goods & services provided.  The choices provided to the urban consumer are exciting & interesting and is a magnet for people.  I believe that there is a growing ambiguity between supply & demand or between service & consumer, I believe that this is a centripetal force as well.

Centripetal Forces (decentralizing) forces included all of the draw-backs of urban living: congestion, pollution, crime, high cost of living/building/transportation, etc.  New technology can also have a centripetal effect; streetcars in the 19^th century, automobiles in the 20^th, and working over the internet in the 21^st century.  Smaller cities may also develop ways to provide more large cities amenities as well, which could act as a centripetal force.