For the urban patterns research, in addition to delineating the urban areas for each year, I wanted to delineate exurban areas beyond the urban areas that could reasonably be considered to be parts of the metropolitan area related to the urban core. Unlike the census Urbanized Areas, however, there is no accepted standard definition for exurban areas. Fortunately, a thorough review of past studies of exurban areas and how they were defined has been provided by Berube and others (Finding Exurbia, Brookings, 2006).
A minimum population or housing unit density–obviously much lower than the urban density threshold–was the most common criterion used in defining exurban areas. Other factors were also considered, especially commuting to the urban area. Data are not available over the entire period of the urban patterns dataset to allow the use of commuting. However, the maximum extent of the exurban area would be limited to the area of the Combined Statistical Area (CSA) or Metropolitan Statistical Area (MSA), which at a minimum guarantees interaction with the urban area for 2010 for the counties as a whole, if not individual tracts.
I decided to define exurban areas as the sets of contiguous tracts that were adjacent to the urban areas and had housing unit densities greater than some value. The minimum density levels used to define exurban areas in various studies varied widely, from 40 acres per housing unit down to about 10 acres per unit. (For studies using the lowest densities, the extent of the exurban areas was most often limited by the commuting criterion rather than density.) I approached the problem by mapping the tracts meeting different minima in 2010 to make a judgment as to what looked reasonable.
The very low minimum density thresholds of 30 or 40 acres per unit frequently resulted in all or most of the CSA or MSA being considered exurban, with the tracts meeting these levels extending far beyond those areas, especially in the eastern U.S. On the other hand, a density minimum of 10 acres per unit produced much smaller exurban areas than seemed reasonable and consistent with personal observation.
The choice came down to thresholds of either 15 acres per unit or 20 acres per unit. The resulting exurban areas generally looked appropriate for most areas. The final choice of 15 acres per unit came down to a number of specific situations where the lower density level produced areas that seemed too large. I’ll give two examples: The exurban area for Indianapolis in 2010 would have extended south at least halfway to Louisville, through area I would never consider exurban. And the Portland exurban area would have encompassed a large portion of the Willamette valley.
A further check reinforced my decision on the minimum density for exurban areas of 15 acres per unit, which is one-fifth of the urban density theshold. For CSAs or MSAs adjacent to other CSAs or MSAs, it was not uncommon for both exurban areas to extend to the common boundary. But for areas not adjacent to others, the extent of contiguous exurban density tracts was generally either confined within the boundaries of the CSA or MSA or extended beyond the boundary at only one or two points, with a string of exurban density tracts along a highway. (This is much like census Urbanized Areas, which frequently have such tendrils of urban development extending outward.) So the density threshold for exurban areas seems consistent with the areas of significant metropolitan interaction as indicated by the CSA and MSA boundaries.
This process of defining the exurban areas is treated in greater detail in the paper, “Defining Exurban Areas for the Analysis of Urban Patterns Over Time” which can be downloaded here.