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U of M researcher offers new understanding of urban sprawl
Research finds new city dwellers spreading out on twice the land of established residents
Across the United States, people moving into cities are settling on twice the amount of land as established residents according University of Minnesota civil engineering assistant professor Julian Marshall. His findings offer a new numerical tool for measuring urban area expansion and are featured in the September 2007 issue of Urban Studies, an international journal on urban planning and policy.
Determining how cities change and grow in response to population increases is a timely question.
"This year, for the first time in history, a majority of people will live in urban areas. In future decades, urban population growth will greatly exceed rural population growth," Marshall said. The desire new residents have for bigger homes and yards leads to even greater implications for social, health and environmental concerns associated with urban sprawl, Marshall explained.
Marshall analyzed U.S. census data on urban land-use and population from 1950 to 2000. "I initially found the patterns by accident, just by playing with the data," Marshall said. He realized that from decade-to-decade, newcomers steadily stretched city borders by occupying double the land of existing residents.
The researcher's discovery was revealed while developing a new way to predict how urban areas grow over time in response to population increases. Making this prediction for a single city can be difficult, and depends on the specifics of that city such as land availability. However, when considering the growth of all cities over time, several patterns emerge. This first pattern is, as noted earlier, newcomers occupy twice the land area as existing residents. Another pattern is that the average number of people in a mile-wide strip of land across a city is constant over time. This second pattern is possible because of a balance is struck between low-density urban growth at the edge of cities and new high-density housing in the urban core.
A third pattern builds on relationships known as "rank-size rules." The idea is that when cities are ranked from largest to smallest, the size of each city follows a predictable proportion. For example, the population of the second-largest city is equal to one-half the largest city, and the population of the third-largest city is one-third that of the largest city, and so on. Marshall identified that similar rank-size rules hold true for other attributes of cities, such as population density and land area.
Marshall's findings provide mathematical descriptions of these observations, and offer predictions regarding city sizes in future decades. Those results predict how the system as a whole behaves - that is, distributions of values for all cities - but not what will happen in any one city.
"The strength of the mathematical associations and the length of time the patterns have
held (for 50 years) is surprising," Marshall said. "I could hardly believe what I found." |
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