social and economic factors, particularly household income, usually exert a stronger influence. 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 smart growth. 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 "Ceteris paribus, urban intensification which increases population density will reduce per capita car use, with benefits to the global environment, but will also increase concentrations of motor traffic, worsening the local environment in those locations where it occurs". 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 has been more successful in this respect. This study also reviewed evidence on the local effects of building at higher densities. At the level of the neighbourhood or individual development positive measures (e.g. improvements to public transport) will usually be insufficient to counteract the traffic effect of increasing population density. This leaves policy-makers with four choices: intensify and accept the local consequences, sprawl and accept the wider consequences, a compromise with some element of both, or intensify accompanied by more radical measures such as parking restrictions, closing roads to traffic and carfree zones. In contrast, the city of Cambridge, Massachusetts reported that its Kendall Square neighborhood saw a 40% increase in commercial space attended by a traffic decrease of 14%. A report by CEOs for Cities, "Driven Apart," showed that while denser cities in the United States may have more congested commutes they are also shorter on average in both time and distance. This is in contrast to cities where commuters face