Building in or out? Examining urban expansion patterns and land use efficiency across the global sample of 466 cities with million+ inhabitants

Expansion of urban areas accompanied by decreasing population density results in inefficient urban growth from a spatial-ecological perspective. This study examines variations in the urban expansion patterns across 466 world cities with million + inhabitants in terms of the contribution of three growth modes (infilling, edge expansion, and outlying) and its relationship with urban land use efficiency, measured in terms of urban density. Focusing on two different time periods (1985–2000 and 2000–2015), it has been found that cities that experience inward expansion are largely located in the Global North, while cities in the Global South experience comparatively more outward than inward expansion. We also find a positive relation between inward expansion and the annual change rate of urban density. However, we hypothesize that this is largely attributed to pronounced variations in the initial per capita land consumption, particularly between cities located in the Global North and Global South. Indeed, when we examine the effect of NDEI on the end-point urban density, and control for the initial urban density, a different conclusion is achieved. We find that in the Global North, more inward expansion of cities decreases urban density compare to the subgroup of cities located in the Global South in which inward expansion amplifies urban density. Understanding such differential effects of urban expansion patterns on urban density is important for our advancement towards sustainable development goal indicator 11.3.1.