Before the development of the steam engine, transport had changed hardly at all for millennia, and even the greatest cities were still gauged by distances which could be comfortably walked. Faster transport was able to release some of the pressure towards excessively high densities, by enabling workers to live further from the factories. The progression in urban development which accompanied these improvements in transport is clearly visible if density is plotted against distance and time. The impact of steam railways (both surface and underground lines) is seen from mid-century onwards as density falls, and the further acceleration around the turn of the century due to electric tramways and underground rail is also clearly marked. The past is well-documented, at least, though even then it is frustratingly difficult to identify and disentangle the various interactions which exist between transport, travel and urban development.