The densities with which urbanised regions are occupied can have a significant impact on energy use and emissions, via the patterns of personal mobility that are enabled and encouraged. The potential for using this variable as a tool for environmental regulation is limited, however, for two inter-related reasons. One is that actual densities are an outcome of complex processes of individual choice over which planners have little direct control. The other is that planning operates only at the margins of physical development, with much slower and more modest impacts on the behaviour of the population as a whole than would changes in relative transport costs, in particular. (c) 2008 Queen's Printer and Controller of HMSO. Published by Elsevier Ltd. All rights reserved.