than sprawling cities, particularly in North America, has been largely accepted – although the relationship is clearer at the extremes across continents than it is within countries where conditions are more similar. Within cities, studies from across many countries (mainly in the developed world) have shown that denser urban areas with greater mixture of land use and better public transport tend to have lower car use than less dense suburban and exurban residential areas. This usually holds true even after controlling for socio-economic factors such as differences in household composition and income. This does not necessarily imply that suburban sprawl causes high car use, however. One confounding factor, which has been the subject of many studies, is residential self-selection: people who prefer to drive tend to move towards low density suburbs, whereas people who prefer to walk, cycle or use transit tend to move towards higher density urban areas, better served by public transport. Some studies have found that, when self-selection is controlled for, the built environment has no significant effect on travel behaviour. More recent studies using more sophisticated methodologies have generally refuted these findings: density, land use and public transport accessibility can influence travel behaviour, although social and economic factors, particularly household income, usually exert a stronger influence. The paradox of intensification Reviewing the evidence on urban intensification, smart growth and their effects on travel behaviour, Melia et al. (2011) found support for the arguments of both supporters and opponents of the compact city. Planning policies which increase population densities in urban areas do tend to reduce car use, but the effect is a weak one, so doubling the population density of a particular area will not halve the frequency or distance of car use. For example, Portland, Oregon, a U.S. city which has pursued smart growth policies, substantially increased its population density between 1990 and 2000 when other US cities of a similar size were reducing in density. As predicted by the paradox, traffic volumes and congestion both increased more rapidly than in the other cities, despite a substantial increase in transit use. These findings led them to propose the paradox of intensification, which states: At the citywide level it may be possible, through a range of positive measures, to counteract the increases in traffic and congestion which would otherwise result from increasing population densities: Freiburg im Breisgau in Germany is one example of a city which