Figure 1 illustrates different patterns of urban areas. The ongoing debate on the environmental benefits of compact urban areas partly originates from the fact that the notion of compactness covers various dimensions. Findings and conclusions may well vary with the studied urban structure indicator, air pollutant, and measure of pollution (i.e. concentration or emission levels). In addition to environmental improvements, continuous urban areas may induce benefits such as energy savings, lower costs of maintenance for energy and transport systems, improvement of quality of life through local services and jobs, and more efficient infrastructure investments (OECD, 2012a). In this respect, a compact urban area, given its proximate development, is expected to produce lower emissions of transport-related pollutants compared to a fragmented urban area (e.g. Urban area 2 and Urban area 4).