The Dynamics of Global Urban Expansion

This study examined the dynamics of global urban expansion by defining a new
universe of 3,943 cities with population in excess of 100,000 and drawing a stratified
global sample of 120 cities from this universe. Population data and satellite images for
two time periods?a decade apart?were obtained and analyzed, and several measures
of urban extent and expansion?among them the built-up area of cities and the average
density of the built-up area?were calculated. Data for 90 cities out of the global sample
of 120 is presented and analyzed in this report. Weighted averages of the built-up area
and the average density, as well as compactness and contiguity measures?and their
change over time?are presented for nine regions, four income groups and four city size
groups covering the entire globe. Densities in developing-country cities were found to
be some three times higher than densities in cities in industrialized countries, and
densities in all regions were found to be decreasing over time. If average densities
continue to decline at the annual rate of 1.7%?as they have during the past decade?the
built-up area of developing-country cities will increase from 200,000 km2 in 2000 to more
than 600,000 km2 by 2030, while their population doubles. Ten econometric models that
sought to explain the variation in urban extent and expansion in the universe of cities
were constructed, and several hypotheses postulated by neoclassical theories of urban
spatial structure were tested. All tests yielded R2 values in excess of 0.80. The policy
implications of the analysis are presented and discussed. The Central message of this
study is quite clear: Developing country cities should be making realistic?yet
minimal?plans for urban expansion, designating adequate areas for accommodating
the projected expansion, investing wisely in basic trunk infrastructure to serve this
expansion, and protecting sensitive land from incursion by new urban development.