Urban density: Promoting sustainable development – Part 1

Density refers to how intensively urban land is utilised; proximity particularly concerns the location of urban agglomerations in a metropolitan area. Cities with high levels of population density can more effectively serve their residents with rapid transit as fewer kilometres of infrastructure are needed to serve the same population. High density (population, housing, jobs) levels are commonly associated with “compact city” strategies. Among several types of urban forms, the compact city has been presented as a way of encouraging urban sustainability as it promotes walkable, eco-friendly urban forms. Globally, cities are becoming denser; this densification accounts for 50% to 60% of the global city population growth. This increase in density requires more investments to provide housing, jobs and services such as transport.

The COVID-19 pandemic has reinforced the value of proximity by enhancing accessibility through urban design and planning. With COVID-19, debates have started to emerge on the vulnerability of densely populated urban areas. They are regarded as places where the risk of contagion is higher than in low-density places. However, research has found that density is not significantly associated with COVID-19 infection rates; in fact, areas with high density tend to have lower death rates. OECD research has concluded that "…it is not density alone that make cities vulnerable to COVID-19, but the structural economic and social conditions of cities make them more or less able to implement effective policy responses.”

Density levels vary depending on the income level of every country. Cities in low-income countries are 4 times denser than those in high-income countries; the population density in cities in North America is less than 2,000 inhabitants per km², whereas in South Asia and Sub-Saharan Africa, it is around 8,000 inhabitants per km2. Moreover, in Europe, cities with similar levels of population have very different densities, reflecting their differences in urbanisation patterns. For example, cities like Milan (73 people/hectare [ha]), Munich (44 people/ha), Prague (25 people/ha), Vienna (41 people/ha) and Warsaw (33 people/ha) with similar population levels differ in their levels of density.

In most metropolitan areas, the majority of the residents live outside the city core and these people are in general not able to access opportunities by public transport nearly as well as residents in central areas. The problem is that urban growth tends to happen outside city centres where there is limited public transport service. Indeed, according to the European Commission and the OECD, the further away from the city centre, the lower the densities are, and the larger the city, the more distance is needed for densities to drop. In the Czech Republic, for instance, the built-up areas of cities have increased in recent years leading to urban sprawl and a process of suburbanisation as more people live in the suburbs than in the core areas. Suburbanisation is one of the causes of high levels of car ownership and in consequence heavy road traffic and air pollution; public transport options are often limited in suburban areas. Greater efforts are needed to allow for and encourage densification, in particular easing density restrictions in low-density areas close to city centres and along public transport corridors; this is key but gradual densification should also be permitted in most parts of an urban area.

Many cities across the world are promoting policies towards more compact urban developments. In a compact city, urban land is intensively used, urban agglomerations are contiguous and there is a clear difference between rural and urban land. Moreover, urban areas are linked by public transport systems that determine how effectively urban land is utilised. Another characteristic of compact cities is that they facilitate access to local jobs and services. For that, land use is mixed, and most residents have access to services and goods either by foot or by public transport. Research suggests that higher population densities, especially when combined with high employment densities, are strongly correlated with easier access to goods, services and information. Conversely, in cities with low densities of employment, commerce and housing, there is generally an increase in the average travel distances for accessing opportunities. These longer travel distances also contribute to higher GHG emissions.

The effective management of density is key to promoting compact, well-planned cities. Densification is perceived as a fundamental strategy for creating sustainable accessibility. The creation of high-density, mixed-use places requires transport investment to be fully aligned with the city’s growth strategy. At a minimum, densification may be possible by easing land use restrictions such as restrictive zoning regulations and planning decisions that prevent it. Explicit restrictions such as floor-to-floor area rations or implicit restrictions such as minimum lot-size requirements and limitations on multifamily homes are just some practices that prevent densification. Policies that promote compact cities tend to incentivise the development of brownfield over greenfield land. Higher density makes it easier for cities to promote mass transit, as it needs high density to pay off investments. Hence, density and mass transit must be planned jointly. Allowing high densities where there is no mass transit or allowing mass transit where there is low density is not likely to lead to better accessibility.

The central goal is to promote high density in city centres where services and goods can be accessed by walking. The strategies may include the establishment of new residential areas or buildings. Some instruments to promote compact city development include: minimum density standards, mixed-use regulation and a density bonus for developers.