Making Room for a Planet of Cities

This research report seeks to enrich our understanding of policies that are appropriate for urban expansion. It looks at three discrete attributes of urban spatial structure and their change over time, namely the average population density in their built-up area; fragmentation (the amount of open space in and around cities that is fragmented by their built-up area); and urban land cover (total land area occupied by cities). This drew on analyses that compared satellite images and urban populations in a global sample of 120 cities circa 1990 and 2000 and a survey of housing conditions and regulatory regimes governing urban expansion in this same sample of cities. It also drew from studies of urban expansion in US cities from 1910 to 2000, of urban expansion in 30 other cities from 1800 to 2000 and from an analysis of a new global urban land cover map of cities with 100,000 or more inhabitants (this data is also available on the web – see the Book Note below on The Atlas of Urban Expansion).

The report summarizes the key research findings:

• On average, built-up area densities in low- and middle-income countries are double those in Europe and Japan, and such densities in Europe and Japan are double those of the United States, Canada and Australia.
• Average built-up area densities declined by 2 per cent per annum between 1990 and 2000 and have been in persistent decline for a century or more.
• Cities have fragmented open spaces in and around them that are equivalent in size to their built-up areas, but the share of fragmented open space within cities declined slowly yet significantly in the 1990s.
• On average, the annual growth rate of urban land cover was twice that of the urban population between 1990 and 2000, and most of the cities studied expanded their built-up area more than 16-fold in the twentieth century.
• At present rates, the world’s urban population is expected to double in 43 years, while urban land cover will double in only 19 years.
• The urban population of low- and middle-income countries is expected to double between 2000 and 2030, while the built-up area of their cities can be expected to triple.

One of the key issues that this work and others by the same authors has highlighted is the need for cities to make room for growing populations. They criticize what they call the “containment paradigm” when it is applied to rapidly growing high density cities – for instance, urban growth boundaries that are too tight-fitting, misplaced hopes on infill, unnecessary densification, over-reliance on regulation and undersupply of arterial roads that can carry public transport. It notes that while containment and densification policies may be suitable in some cities where population growth has slowed and densities are too low to sustain public transport, this approach should not be applied universally. In discussing the need for each growing city to have realistic projections for urban land needs, it notes how the plans for the expansion of New York City in the early nineteenth century and for Barcelona in the mid-nineteenth century made such provision. It also suggests that generous but credible provision for land for expansion will avoid speculation and high land prices. In terms of open space, the approach is not to constrain a city’s growth with a greenbelt but for built-up areas and open spaces to inter-penetrate each other as the city expands outwards. The paper also discusses the additional need for an arterial road and infrastructure grid which, it suggests, will keep down land for housing costs, allow the city government rather than developers to shape expansion, provide the basis for high quality public transport and lower infrastructure costs. It emphasizes that the “making room paradigm” does recognize the importance of markets in the development of urban land for residential, economic and civic activities but it also recognizes that markets do not ensure the creation of public spaces or esta