Serious concerns with accelerating global warming have been translated into urgent calls for increasing urban densities: higher densities are associated with lower greenhouse gas emissions, especially those related to vehicle kilometers traveled (VKT). In order to densify meaningfully in the coming decades, cities need to make room within their existing footprints to accommodate more people. In the absence of adequate room within their existing footprints, cities create more room through outward expansion, typically resulting in lower overall densities. We introduce a quantitative dimension to this process, focusing on the population added to a global stratified sample of 200 cities between 1990 and 2014. In three-quarters of the cities we studied, the areas built before 1990 gained population and thus densified significantly. On average, however, only one-quarter of the total population added to the 200 cities in the sample in the 1990–2014 period were accommodated within their 1990 urban footprints, while three-quarters were accommodated within their newly built expansion areas. That resulted in an overall decline in average urban densities during the 1990–2014 period despite the near-global, decades-old and rarely questioned consensus that urban expansion must be contained.
Serious concerns about global warming have been translated into urgent calls for increasing urban densities, as higher densities are associated with lower carbon emissions from both vehicles and buildings. However, attempts at effective densification have generally failed and urban densities continue to decline in cities across the world. Calls for densification without making room for it have resulted in serious housing supply bottlenecks in many cities and have rendered their housing unaffordable. If affordable densification is to be successful, it is necessary to understand the factors that constitute urban density. A novel way is presented for factoring the average density of cities into constituent factorsthree or seven factors-that when multiplied together reconstitute urban density. This factoring methodology is presented together with the preliminary measurement of these factors in 10 cities in 10 world regions. This approach allows, for the first time, a clear understanding of how different cities acquire their density: Hong Kong gets its density from building height; Kinshasa from crowding; and Dhaka and Bogotá from residential coverage. This anatomy of density offers a new outline for a comprehensive strategy for city densification: one that addresses each and every one of the factors that constitute urban density. PRACTICE RELEVANCE Urban density is usually defined as the ratio of the total population of a city and its total area. This is the most appropriate single metric for measuring progress in densification in cities and is now a central objective of the global climate change agenda. However, this metric is rather crude and often hides more than it reveals. The anatomy of density offers a new, simple method for factoring urban density into its constituent factors that when multiplied together reconstitute urban density. This approach offers city leaders and urban planners a new way to consider and develop comprehensive policy options and strategies for city densification that addresses each and every one of the factors that constitute urban density and the tradeoffs between them.
Recent research indicates that a simplified approach to urban planning in Sub-Saharan African cities can address the challenges of rapid urban growth. Current plans focus too heavily on the existing area of the city and offer unrealistic agendas for future urban growth, such as densification, containment and high-rise development; plans that are often too complicated and too costly to be deployed in a developing-world context. In response, New York University and the Government of Ethiopia have created a programme to deploy a simple methodology called Making Room for Urban Expansion in 18 Ethiopian cities that are experiencing rapid growth. The programme is called the Ethiopia Urban Expansion Initiative. The Initiative set aside a number of standard planning objectives and instead focused only on expanding city boundaries to include adequate land for expansion, designing and protecting a network of arterial roads spaced approximately 1 km apart, and identifying and protecting environmentally sensitive open spaces. These efforts focused on areas that had not yet been occupied by development. This article reports on the preliminary results from the four Ethiopian cities participating in the Initiative that began in 2013. The results from the first four participating cities show that simple plans can lead to the creation of new arterial roads, increasing access to peripheral land and potentially bringing the available land supply in line with projected growth. These activities can be done at the local level and implemented with limited support from consultants and from the regional and national government, and it requires minimal public investment.
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