# **1. Introduction**

Over the last six decades, Sub-Saharan Africa (SSA) has been the world’s fastest urbanizing region. Between 1960 and 2021, the share of its population living in officially classified urban areas increased almost 15-fold from around 33.3 million to just over 494 million, while the share of its population living in such areas climbed from 14.6 to 41.8 percent.<sup>1</sup> This rapid urbanization is, moreover, set to continue in the decades ahead. By 2050, more than 1.26 billion people will be living in SSA’s towns and cities, more than 2.5 times the number today. Such rapid urbanization carries with it important economic, social, and environmental implications for the development of a region in which more than one-third of the population lived in extreme poverty in 2019.<sup>2</sup> Despite its rapidity and importance, however, remarkably little is known about the detailed anatomy of patterns of urbanization across SSA countries. This neglect reflects both a lack of detailed official data on urban settlements and populations, and, partly also related to this, a relative lack, with some notable exceptions, of research interest among urban economists in the region’s urbanization.<sup>3</sup> Against this backdrop, this paper aims to provide a detailed descriptive analysis, for the year _circa_ 2015, of patterns of urbanization across SSA countries. In doing so, the paper distinguishes between two types of urban area – cities and towns. To identify these areas, the paper applies a modified version of an algorithm first developed by de Bellefon et al. (2021) to high-resolution gridded population data for the region, which is itself derived from digitized maps of the footprints of _all_ buildings in the region from very high-resolution satellite imagery. This allows for a consistent definition of urban areas across all countries in the region, thereby overcoming the measurement problems that arise from relying on official definitions of urban areas, which vary widely across both SSA countries and countries globally (World Bank 2009, Roberts et al. 2017, Dijkstra et al. 2021).

As part of its analysis of urbanization for SSA, the paper presents evidence on key features of the urban landscape, both across and within urban areas. Looking across urban areas, the paper provides evidence on, _inter alia_ , aggregate differences in levels of urbanization between countries, numbers of urban areas, the share of land they occupy, their population densities, and the structure of urban hierarchies as captured by, for example, Zipf’s law, as well as on how these features of urbanization correlate with both a country’s size (i.e., population) and level of development. Meanwhile, looking within cities, it provides evidence on key empirical regularities such as population density gradients and the number of (sub-)centers.

The paper contributes to three interrelated literatures. First, it contributes to a nascent literature that provides new evidence on patterns of urbanization in Africa by leveraging satellite data. This includes a recent paper by Henderson, Peng, and Venables (2022) which uses data from the European Union’s Global Human Settlements built cover data set, _GHS-BUILT_ , to examine patterns of settlement growth across SSA between 1975 and 2014. Henderson et al. identify a rapid growth in the number of settlements – from about 47,500 to around 111,500 – with evidence of complementary (competitive) patterns of growth for nearby settlements of different (similar) size.<sup>4</sup> They also empirically reject Gibrat’s law, which underpins Zipf’s law in Gabaix (1999), finding that built-up area growth has been fastest for the settlements that were initially smallest in 1975.

> 1 The statistics on urbanization cited in this paragraph are based on data from the United Nations’ World Urbanization Prospects: 2018 Revision (https://population.un.org/wup/).

> 2 Extreme poverty is defined based on the global extreme poverty line of $2.15 a day (2017 Purchasing Power Parity exchange rates). Data on extreme poverty is from the World Bank’s World Development Indicators (https://databank.worldbank.org/source/world-development-indicators).

> 3 Until relatively recently, most empirical urban economics research has been focused on the US and other developed countries, as well as on a few large developing countries such as Brazil, China, India, and Indonesia. Exceptions to the general neglect of SSA countries include several papers by Vernon Henderson and co-authors (Henderson and Turner, 2020; Henderson, Regan, and Venables, 2021; Henderson, Nigmatulina, and Kriticos, 2021).

> 4 Henderson et al. do not explicitly distinguish between different types of settlement (i.e., cities versus towns) but instead examine settlement growth in general based on a binary classification of built-up versus non-built-up.

2