 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 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.
Professor 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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