The Persistent Decline in Urban Densities
Using satellite imagery, census data and historical maps, we report on density variation among cities the world over. We find significant differences in the average population density in the built-up areas of a global sample of 120 cities: In 2000, average density was 28±5 persons per hectare in cities in land-rich developed countries, 70±8 in cities in other developed countries, and 135±11 in cities in developing countries. We also find that built-up area densities in this sample declined significantly, at an average annual rate of 2.0±0.4 percent, between 1990 and 2000. We report on the five-fold decline in average tract density in 20 U.S. cities between 1910 and 2000, at an average long-term rate of 1.9 percent per annum, on the slowing down of the rate of decline in recent decades, and on the decline in several other density metrics during this period. Using historical maps and historical demographic data for 1800–2000, we also report on the threefold decline in average urbanized area densities in a global sample of 30 cities during the twentieth century, following an increase in average density in the nineteenth century. On average, densities in this historical sample have been in decline since their peak circa 1890±16, at an average long-term annual rate of 1.0-1.5 percent. All or most of the significant factors accounting for density variations and density decline are identified in multiple regression models and the implications of the findings for urban containment and compact city strategies in different regions are examined. At current rates of density decline in the cities of developing countries, for example, when their urban populations double in the next 30 years, as now expected, their built-up areas will likely triple. Minimum preparations for this massive expansion are clearly in order.