Curbing greenhouse gases emissions from transport in cities is a major challenge for climate policies. The pioneering works of Newman and Kenworthy suggest that many of these emissions could be avoidable, since they result from city densities and the share of road traffic in urban transport. However, implementing a change raises difficult questions. The respective roles of transport and land use policies are controversial. There is an intense debate about priorities between proponents of vehicle efficiency improvements, advocates of abrupt changes in mobility behaviours, and heralds of public transport.

In Europe, the sustainable city is generally seen as the inversion of the urban sprawl process, coupled with the densification of inner suburbs thanks to new public transport infrastructure. Our analysis provides an argument in favour of this scenario, by noting that cost-benefit assessments of these projects should take into account the benefits of induced changes in urban densities, even for marginal extensions.

But all factors impacting urban sprawl must also be properly valued. A stylised example of the model shows that the view a single pre-determined type of post-carbon city is thus far too simplistic.