In the past decade, one-third of the OECD metropolitan areas in Europe, Japan and the United States have continued increasing their built-up areas, at a pace even faster than population growth. The urban sprawl index measures the percentage change overtime in the built-up area "available” per person living in a metropolitan area. Values of the index higher than 1 mean that the growth of built-up area was faster than the growth of population (or in other words the built-up area per person has increased).