As urban areas become denser and rely more on public transport, walking and cycling, carbon emissions tend to decrease. The urban form of cities is not the only determinant of lower per capita emissions, but it is a critical factor among others, such as energy prices, the productive structure of urban areas or the public transport networks (OECD, 2011a). Figure 2.2 shows that when population density in built-up areas is high, motor vehicle ownership tends to be lower. Urban density is calculated based on the OECD definition of “predominantly urban” areas.