population and employment distributions within metropolitan areas (Giuliano et al. 2008). Nor do they provide a sufficiently detailed picture of the rich urban landscape. Outside the central city, density levels can vary greatly, from the generally more dense inner suburbs, to the very low densities of many outer suburbs, to housing complexes and communities of varying densities in between—all with different implications for travel and trip making.

In the field of economic geography, special attention has been paid to the location of employment, leading to the characterization of employment in metropolitan areas as monocentric, polycentric, or noncentered or dispersed (Lee 2007). The monocentric model has increasingly lost its explanatory power as employment has decentralized and the reasons for clustering in a single CBD have diminished (see Clark 2000 in Lee 2007). Two competing views have emerged with regard to the implications