Reflections on Metropolitan Form and Binary Cities

Accessibility is not purely a field effect, but also a network effect. Rather than the center broadcasting a field effect, the network broadcasts a field of effect, with the strength of that field proportional to the network time-distance to the center. 

Take Binary Metropoli such as Dallas-Ft. Worth or Minneapolis-St. Paul, where two existing monocentric cities have become conjoined. The patterns of sub-centers is distinct from emergent polycentric, where monocentric cities have developed sub-centers. In monocentric cities with radial transit routes, sub-centers emerge as  function of distance from the center. As sub-centers develop sufficiently that their level of activity density warrants it, orbital routes between peripheral sub-centers will develop. For binary metropoli, the pattern is different. The intersection of radial branches between the city centers should emerge as a sub-center.

How does a sub-center differ from an 'Edge City'? Sub-centers have to do with the efficient allocation of retail and services minimize the overlap in market areas, and minimize transportation travel time to the centers.

To use the 'hierarchy of retail' as an example:

• Regional malls...down to neighborhood,
• Community Center/Power Center -- big box anchored, 100,000 SF 
• Neighborhood, 55-60k SF..
• Grocery anchored
•  Specialty/In-line (Strip malls)
• C-Store

The distribution is inversely proportional to their frequency. 

Edge Cities, in contrast, contain more than retail. Garreau characterized them in terms of office development. High-rise office development was traditionally the purview of downtowns, and its emergence outside them was kind of scandalous. High rise office development is associated with higher order services--not personal services and retail, but what is often called "Producer Services", or "Professional Services", characterized by FIRE industries (Finance, Industry, Real Estate).