More on the sizes of exurban areas

The post before this one described the tremendous variation in the sizes of exurban areas surrounding large urban areas in the United States. And since exurban area land area and housing units are strongly related to those quantities for the urban area, looking at the ratios of exurban to urban size proved useful for considering the relative sizes of exurban areas.

The relative sizes of the exurban areas as measured by these ratios varied greatly across the census regions. The mean ratios in the Northeast were the largest for both land area and housing units, and exurban areas in the South were nearly as high. Exurban areas in the West were on average the smallest in relative terms, with ratios around one-third of those for the Northeast. The Midwest exurban areas fell in the middle.

This raises the question as to why exurban areas were so much smaller in the West. Ironically, the clue comes from the area having the smallest exurban area in relative terms, Miami-Fort Lauderdale-West Palm Beach. Expansion of the urban and exurban areas is constrained by the Atlantic Ocean to the east and by the Everglades to the west. The urban area has expanded to include most of the available land, drastically limiting the size of the exurban area.

So barriers to urban and exurban expansion can limit the size of exurban areas (note that I am not the first to raise this possibility). Many areas in the West have various barriers to expansion. These include mountains and federal lands not available for development. Also, the arid climates and the dependence on water often brought in from a distance by centralized agencies can mean that water utilities are only expanded at the edge of the existing urban area to serve new urban development.

I subjectively identified those urban and exurban areas for which expansion was significantly constrained by mountains or federal lands. For arid climate, an objective measure could be used, areas that received average annual rainfall of less than 20 inches. Note that the areas facing these barriers were all in the West region with the exception of El Paso. While that area is in Texas, in the South, it is literally as far west in the South region as possible. Comparing the mean exurban size ratios for areas with and without these barriers, the differences were large and highly significant. But the areas with mountains, federal lands, and arid climates overlap to a very high degree, so it is impossible to distinguish their effects.

Looking at the areas with arid climates, the one objective measure, the average ratio of exurban to urban land area for the arid areas was about 40 percent of the ratio for the non-arid areas. For housing units, the arid ratio was about one-third the non-arid ratio. Using simple regression models to predict exurban land areas and housing units yielded these differences: Areas with arid climates had exurban areas that were on average 934 square miles smaller and had 120,000 fewer housing units.

More information on the exurban areas in 2010 can be found in my paper, “Exurban Areas Around Large Urban Areas in the U.S. in 2010,” which can be downloaded here.


Houston’s urban growth and Hurricane Harvey

Hurricane Harvey has dumped massive amounts of water on the Houston area, causing widespread, calamitous flooding. Some context on the tremendous growth of Houston can show how many more people are at risk from hurricanes now than from hurricanes that have periodically struck that general area in the past.

Reports from Houston have described Houston as the fourth largest city in the United States with a population of 2,300,000 (estimated for 2016). As urbanists understand, this will always be just a portion of urban area population. Some reports have mentioned the size of the metropolitan area (MSA), which is much larger. But metropolitan areas include substantial exurban and rural territory outside of the built-up urban area. Certainly the populations included in both areas are at risk from hurricanes. But the built-up urban area is of special significance because urban development exacerbates the risks of flooding due to increased impervious surface and man-made structures that retard runoff. And flooding in urban areas poses different types of risk (not necessarily more severe) than flooding in rural areas.

My focus here is on what I have defined as the Houston urban area in my urban patterns research. This is an effort to delineate the more-or-less built-up portions of the larger metropolitan area for each census year from 1950 to 2010. These urban areas are similar to the census Urbanized Areas that delineate the area and population officially considered to be “urban,” but have the advantage of being defined in a consistent manner over time (along with other research advantages given that they are defined using consistent areal units).These urban areas consist of the contiguous census tracts having at least the urban density of 3 acres per housing unit or 213.33 housing units per square mile. This is generally equivalent to the census threshold of 500 persons per square mile currently used to delineate Urbanized Areas. (For more detail, see this earlier blog post.)

First, I’ll show maps of the extent of the Houston urban area in 1950, 1980, and 2010, showing the tremendous expansion:

Houston urban area 1950

Houston urban area 1950

Houston urban area 1980

Houston urban area 1980

Houston urban area 2010

Houston urban area 2010

Now for some basic statistics describing the urban areas at these three points in time. This table shows the land area of the urban area, the number of housing units, and the housing unit density for each census year:

Year      Area (sq mi) Housing Units         Density
1950 167 157,687 945
1980 726 874,881 1,206
2010 1,765 1,882,352 1,066

The Houston area grew 10 times larger in terms of land area, becoming the ninth largest urban area in 2010. Surprisingly it had the fourteenth largest land area in 1950 despite its much lower ranking in terms of housing units because many other urban areas were far denser at that time. This demonstrates the vast increase in the land area that could be vulnerable to urban flooding now as opposed to the middle of the last century. And the much greater impervious surface area and amount of structures that would contribute to greater flooding.

Housing units increased by 12 times. Houston was tenth among the large urban areas in 2010 in terms of housing units while it was only twentieth in 1950. So again, a huge jump in the number of dwellings, households, and people at risk in urban Houston. And from 1980 to 2010, the population of the Houston urban area increased from 2,100,000 to almost 4,900,000. (Due to the nature of the data sources used, I do not have population figures for 1950.)

The housing unit density did not change a great deal over the sixty-year period, increasing somewhat from 1950 to 1980 and then dropping back a bit to 1,066 housing units per square mile in 2010. This density was almost exactly the mean housing unit density for the 59 large urban areas in my study in 2010, which was 1,080 (and was even closer to the median value of 1,055). This is in contrast to Houston’s density in 1950, which was forty-second out of 59 with a value of 945 units per square mile compared to the mean for all areas of 1,268. Many other areas saw decreases in density over this period; Houston, with a modest increase, did not. (See this post and this paper for more on changes in density for large urban areas over this period.)

Sizes of exurban areas vary tremendously

A post made some time ago describes my definition of exurban areas for the urban patterns research and the previous post refers to a paper in which I address this in greater detail. These are areas of contiguous census tracts with a minimum density of one housing unit per 15 acres around the 59 large urban areas.

After delineating the areas, I began by looking at the sizes of the exurban areas in 2010. The variation of exurban area sizes was amazing. In terms of land area, the exurban areas ranged from 171 square miles to nearly 7,800 square miles. Numbers of housing units went from 19,000 for the smallest exurban area to 1.2 million for the largest. The smallest exurban area was around the Albuquerque urban area and the largest was New York. Of course some of the variation is due to differences in the sizes of the metropolitan areas. But it is noteworthy that San Diego, not a small area, had the third smallest exurban area.

So perhaps it is more reasonable to compare the areas by looking at the ratio of their land areas and housing units to the areas and units in their urban areas. The smallest ratio for exurban land area to urban land area was about one-third, for Miami. Knoxville was the largest exurban area relative to its urban land area, nearly 8 times as large. Miami likewise had the smallest number of housing units relative to its urban area, 3 percent. And Knoxville had 1.4 times as many exurban housing units as urban units.

The contrast can be seen in the maps of the Miami and Knoxville urban and exurban areas. Also included is Portland, which had the median ratio of exurban to urban land area of about 23 percent. (The three maps are at approximately the same scale.)


Urban and exurban areas for the areas with the smallest, median, and largest ratios of exurban land area to urban land area

Urban and exurban areas for the areas with the smallest, median, and largest ratios of exurban land area to urban land area

More information on the exurban areas in 2010 can be found in my paper, “Exurban Areas Around Large Urban Areas in the U.S. in 2010,” which can be downloaded here.

Defining exurban areas–an update

About 6 months ago, I wrote a post describing how I went about defining exurban areas for my urban patterns research. As I started to do the analysis and prepare for the writing of research papers on these exurban areas, I realized that the question of how I had defined these areas was sufficiently important that this deserved extended treatment in a paper. In the paper, I consider the literature addressing prior attempts to at defining exurban areas and present the logic underlying my approach in greater detail. The paper also includes a far more detailed and formal analysis of the extent to which the 2010 areas of contiguous exurban density are contained within the 2010 Combined Statistical Areas and Metropolitan Statistical Areas.

The paper is entitled “Defining Exurban Areas for the Analysis of Urban Patterns Over Time.” It is on the Research page and can also be downloaded here.

On the pursuit of efficiency in different types of retailing

Many forms of retailing have experienced transformations aimed at increaising efficiency, especially increasing the size of stores and reducing customer service. As I thought about this, I realized that these changes happened at very different times for differnet types of retailing. I decided it would be interesting to look at the differences in timing.

For general merchandise, the department store came as the very early innovation in the mid–19th century, with large stores offering very broad ranges of goods. But it took a century before the self-service discount department store emerged with very limited assistance within the store and customers paying up front. I looked up some dates and found an amazing coincidence: Walmart, Kmart, and Target were all founded in 1962.

For groceries, the order of innovation was reversed. Piggly Wiggly was established as the first self-service grocery in 1916. But like other grocers at the time, it did not carry items such as meat and produce. The first “true” supermarket, King Kullen, did not arrive until 1930. (This was established by research conducted by the Food Marketing Institute and the Smithsonian!)

Looking at more specialized forms of retailing, Toys “R” Us was early, having been founded in 1957. Home Depot opened its first two home improvement superstores in 1979. Lowes, their largest competitor, was founded decades earlier but did not adopt the big-box format until forced to by the competition from Home Deport.

The coincidences of several large-format retailers being founded at about the same time was not limited to discount department stores. Office Depot and Staples were both established in 1984, with Office Max coming only two years later. Best Buy was originally formed as a specialty audio retailer. It changed to its current name, expanded into other types of consumer electronics, and opened its first superstore in 1983. Circuit City followed a similar evolution, changing its name and expanding to become a general consumer electronics retailer a year later. PetSmart was founded in 1985. Petco started much earlier as a mail-order business. I could not find when they first opened physical stores.

I find it fascinating that the big box stores in so many categories started around the same time in the 1980s. Why then? The efficiency of large self-service stores had been established far earlier with supermarkets and discount department stores. Toys “R” us started as a specialized “category killer” retailer over two decades earlier, and the example of Home Depot had been around for a number of years. And suddenly retailers in very different categories moved in that direction at about the same time.

Economies of scale in retailing

In the previous post, I discussed how economies of scale, enabled by improvements to transportation, led to the development of segregated land uses in the 19th century. Now I’d like to focus on the various economies of scale in retailing and their implications.

The obvious benefits of larger stores and the ability to take advantage of economies of scale accrue to the retailer, of course. The larger store will likely require fewer employees per volume of goods sold. There may also be other efficiency benefits relating to things like inventory. A wider range of goods can be stocked, making the store more appealing to customers.

Economies of scale can also benefit the customers. Wider selection is in most instances a plus. I don’t know how my daughter could have constructed her science fair project, a closed-circuit wind tunnel, without Lowes and Menards. And to the extent larger stores and the attendant economies of scale reduce costs and result in lower prices, customers benefit.

Larger stores selling more goods require a larger market and as a result they will be more widely spaced. It might have been possible to have neighborhood hardware stores, but there cannot be a comparable neighborhood Home Depot.

Beyond these internal economies of scale, external economies of scale or agglomeration economies are also very important for retail land use patterns. Some types of retailers choose to cluster with other similar outlets because customers are attracted by the opportunity to comparison shop. Other businesses locate near larger retailers to take advantage of the customer traffic they generate. This can be within a development, whether it is a large shopping center anchored by department stores or a smaller neighborhood center anchored by a supermarket. Or it can take place simply by locating in the vicinity of the larger, traffic generating businesses, whether in central business districts or outlying retail areas. In either case, the product is larger areas of segregated land uses.

Segregation of land uses is not new

A great deal of (negative) attention has been devoted to the segregation of land uses in newly developed suburban areas in recent decades. The critique is that the development of exclusively residential neighborhoods and the segregation of commercial activities reduces opportunities for walking, requiring increased automobile use. This is sometimes portrayed as a recent phenomenon, bought on by the widespread use of the automobile.

Some perspective is in order, however. Land use segregation is hardly a product of the latter part of the 20th century. The original cause was not the use of the automobile (though transportation was critical). Rather, the initial separation of land uses in American cities dates to the 19th century.

The pre-industrial walking city at the start of the 19th century had very limited separation of different land uses. Given that interaction was limited by reasonable walking distance, different activities just could not be located that far apart.

As the industrial city emerged in the 19th centure, this changed as enterprises sought to capture the advantages of economies of scale and was made possible by improvements in transportation within the city. First the omnibus, then the horsecar, and then electric streetcars and mass transit increased the distances people could travel to work and shop. Factories increased in size and formed increasingly large industrial areas. Larger enterprises required management by concentrations of office workers. The department store emerged to provide a previously unseen variety of goods to shoppers from throughout the urban area. The offices, department stores, and related retail formed the new central business districts, another area of largely segregated land uses.

Of course not all types of establishments saw these increases in scale in the 19th century. For grocery stores, the changes came later. But this was the start of increasing sizes of enterprises, made possible by improvements in transportation, forming areas of segregated land use.