Published on Let's Talk Development

When is a place urban? Applying a new method for defining urbanization to Africa

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A boat traveling down a river. | © Musa Dhlamini / Unsplash A boat traveling down a river. | © Musa Dhlamini / Unsplash

The answer to this, apparently simple, question varies widely across countries. For example, while Angola defines urban areas as “geographic areas with a high population density and concentrated population groups with a high level of infrastructure,” Benin defines them as “localities with 10,000 inhabitants or more.”

While it is understandable that definitions may vary across countries given differences in local context, this lack of comparability poses difficulties for international comparisons. This is particularly so for Sub-Saharan Africa (SSA), where little is known about the detailed anatomy of patterns of urbanization despite its projected rapid urbanization in the decades ahead. In a recently published working paper, we apply a new method—the “dartboard” method—for the consistent delineation of urban areas across 47 countries, and analyze the resulting data to tease out key stylized facts.

What is the “dartboard” method?

The dartboard method draws the urban-rural line based on statistical criteria.  Imagine taking a country such as Rwanda, randomly redistributing its population over its territory, and repeating this exercise 5,000 times. This would generate a counterfactual distribution of Rwanda’s population under the assumption of randomness. By comparing its actual spatial distribution of population with this counterfactual distribution, we could identify geographic areas in Rwanda that have densities statistically greater than expected––“urban” versus “rural” areas. 

This is the essence of the dartboard approach. Moreover, by applying a similar approach, we can, among urban areas, further distinguish between cities and towns, where a city is an urban area that possesses a statistically significant “urban core” of high, relative to urban areas now rather than the overall country, of density.

Five key findings emerge from the application of the dartboard method to Sub-Saharan African countries:

  1. Sub-Saharan Africa is more urbanized than commonly realized.  When we plot the share of a country’s population that lives in “dartboard” urban areas against the share of its population that is officially urban, we see that, with the exceptions of Cape Verde (CPV) and São Tomé and Príncipe (STP), all countries lie above the 45-degree line (Figure 1). Therefore, for 45 out of the 47 SSA countries analyzed, official statistics understate the number of people living in areas that can be considered urban.

Figure 1: Scatterplot of share of people living in urban areas—dartboard versus official

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A scatter chart showing Figure 1: Scatterplot of share of people living in urban areas?dartboard versus official
Note: Calculations based on data for 47 SSA countries. Data on official urban population shares (for 2022) is from the United Nations World Urbanization Prospects: 2018 Revision database (https://population.un.org/wup/).
  1. Most of Sub-Saharan Africa’s urban dwellers live in dense cities rather than towns.  On average, across the 47 SSA countries, 55 percent of people live in cities that have an average density of 1,383 people per km2, while 19 percent live in towns that have an average density of 536 people per km2. Furthermore, within cities, nearly 70 percent of people, on average, live in even more densely populated cores. Hence, even within cities, population is extremely concentrated. Such concentration may contribute to excessive congestion and overcrowding in SSA cities, making the cores important geographic targets for policies aimed at boosting competitiveness and enhancing urban livability.
     
  2. Unlike in developed countries, built-up area in Sub-Saharan Africa more than doubles with urban area population.  For urban areas globally, previous research has estimated that every ten percent increase in a city’s population is accompanied by a 6 percent increase in its built-up area. This is consistent with cities growing through outward expansion and vertical development. However, for SSA countries, we find that every ten percent increase in an urban area’s population is associated with a more than ten percent increase in its built-up area. This is consistent with these areas instead expanding mainly through sprawl, with negative consequences for both their productivity and sustainability.
     
  3. Sub-Saharan countries do not appear to suffer from excessive urban primacy. Excessive primacy occurs when “too many” of a country’s residents live in its largest city because of, for example, political favoritism in the allocation of resources to the capital city.  However, primacy in SSA countries is, on average, less than in the world’s high-income countries (Figure 2). Although this result is based on official urban definitions for high-income countries, it also holds when comparing SSA countries with urban areas in France, for example, as defined using the dartboard method. Hence, concern that SSA’s largest cities may be “too big” seems misplaced.
  1. Most Sub-Saharan African cities are monocentric (i.e., have a single dominant population center with density that tends to decline smoothly with distance). This contrasts with a polycentric city in which there are multiple centers.  As cities expand in size, the evolution from a monocentric to a polycentric urban form may help to ease congestion and bring people closer to opportunities. In SSA, however, most cities have yet to reach this stage of maturity. Hence, although there are exceptions, such as Nairobi in Kenya and Addis Ababa in Ethiopia, for 90 percent of SSA countries, 90 percent of cities have a single center. Whether SSA cities would benefit from planning that helps to facilitate the emergence of more centers is an important question for future research.

By providing a consistent approach to defining urban areas in Sub-Saharan Africa, the dartboard method helps produce new insights into patterns of urbanization in a region that is home to a large share of the world’s extreme poor.  Future work could extend our analysis to provide a more detailed analysis of the dynamics of SSA’s urbanization over time.


Note: This blog is one of a two-part series. The second blog in the series describes accompanying research that combines the delineations of urban areas obtained from the dartboard approach with household budget survey data to reveal new insights into urban poverty.

Authors

Mark Roberts

Lead Urban Economist, World Bank

Pierre-Philippe Combes

CNRS Professor, Sciences Po, Paris

Clément Gorin

Associate Professor of Economics, the University Paris 1 Panthéon-Sorbonne

Shohei Nakamura

Economist in the Poverty and Equity Global Practice

Benjamin Stewart

Geographer, Geospatial Operational Support Team, World Bank

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