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C h a p t e r 7 Measuring and Modeling Global Urban Expansion Stephen Sheppard From the perspective of global urbanization, the first decade of the twenty-first century will be seen not as the time when the problems associated with urbanization first became apparent (these have been widely understood since the nineteenth century and perhaps earlier), nor as the time when the basic outlines of policy solutions first attained widespread recognition, if not agreement. The importance of infrastructure provision, the need for legal institutions that recognize and seek to internalize the externalities (positive and negative) intrinsic to highdensity settlements, and the connection of housing provision to human health and social order have all been understood and discussed to some degree since the cities of the classical civilizations. What sets the present time apart are two facts: one is that for the first time in history these policies directly affect the welfare of a majority of humans living on earth. The other is that, for the first time, we have at our disposal—in the form of remotely sensed data and some basic theories of urban form—the opportunity to measure and model urban expansion on a global scale. The first of these facts is a central rationale both for increasing concern among policymakers and for publication of the present volume . The second is the central focus of this chapter.1 Like many other economic and public policies, urban development policies have often fallen prey to the tendency to assume that there must be a ‘‘local’’ explanation for what would be better understood as part of a global phenomenon. For example, internal migration from the countryside to an urban area increases the demand for housing and results in higher prices for dwellings in the city and conversion of land from nonurban to urban uses in and around the city. This in turn leads to demands for extension of infrastructure provision and complaints about loss of access to open space. This story has been repeated (and documented ) in hundreds of places ranging from seventeenth-century Lon- 108 Urban Spatial Growth and Development don to twenty-first-century Beijing. Frequently, the explanation is that the unique attractions of the particular city under consideration relative to the rural origins of the migrants is the source of the problem, and a variety of rural development strategies coupled with building restrictions in the cities is offered as a solution. These are almost never successful in halting the urban expansion, although they may impose considerable costs on the citizens and generate a variety of rents to be dissipated throughout the economy. In other regions where internal migration is less important or nonexistent , different local explanations become the focus of concern. There are many cities in North America and Europe whose populations have been stable or declining for several decades, but where new structures continue to be built on what was nonurban land. Perhaps the most obvious explanation for this is simply that land is a normal good and in these cities real household income has continued to increase. As their incomes rise, households purchase more of many different goods, but in particular they seek to purchase more housing and more land. This leads to lower-density, expanding cities even when population is constant or slightly declining. Because the policy implications of this explanation are not particularly palatable—since few would support putting the brakes on income growth—alternative local explanations are sought: road building and automobile ownership are identified as likely culprits, along with particular types of commercial development or general ‘‘white flight.’’ Numerous campaigns and local organizations have come together to oppose road building, discount retailers, and a variety of other proposed causes of urban expansion. From a policy perspective, the difficulty with such localized explanations is not so much that they are false (although they sometimes are) as that they are only partly true. Road building and other factors that reduce transportation costs probably do encourage urban expansion, as does population growth from internal migration. Understanding urban expansion requires that we be able to explain very different outcomes that emerge from apparently similar situations, as well as similar outcomes that emerge from very different situations. For example, Ibadan, Nigeria, and Seoul, Korea, have been experiencing urban expansion during the period of 1990 to 2000 of about 2.5 percent per year. Per capita GDP in Nigeria during this period was stagnant or declining slightly, but urban population grew at nearly 3 percent per...

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