Many American cities, described commonly as urban cores, are functionally more suburban and exurban, based on urban form, density, and travel behavior characteristics. Data from the 2010 census shows that 42.3 percent of the population of the historical core municipalities was functionally urban core (Figure 1). By comparison, 56.3 of the population lived in functional suburbs and another 1.3 percent in functionally exurban areas (generally outside the urban areas). Urban cores are defined as areas that have high population densities (7,500 or per square mile or 2,900 per square kilometer or more) and high transit, walking and cycling work trip market shares (20 percent or more). Urban cores also include non-exurban sectors with median house construction dates of 1945 or before. All of these areas are defined at the zip code tabulation area (ZCTA) level, rather than by municipal jurisdiction. This is described in further detail in the "City Sector Model" note below.
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