y utilisation; and infrastructure improvements in rail/public modes would have a noticeable and (relatively) significant marginal impact on the need for road infrastructure requirements. There is, however, no certainty that the political constellation(s) will support such a holistic policy package - not least as many decisions must be jointly enacted by different levels of governments and require considerable time to build policy consensus. Spatial distribution/population density Perhaps the most difficult policy to gain political approval involves (more compact) spatial distribution and (higher) population density - especially within/ between major urban areas. Besides the difficulties of achieving public consensus and political co-ordination, there is the path dependency of urban forms. If infrastructure is best viewed as a system, there is no more complex system than the urban form - comprised of overlapped multi-generational, life-cycle decisions related to migration, urban location, housing choice, employment opportunities, transport choices and decisions, lifestyle choices, etc. The current spatial urban forms of cities as diverse as London, Paris, New York, Houston, Sydney, Tokyo, Hong Kong, etc. have developed over centuries, and while they can be shaped/ INFRASTRUCTURE TO 2030: TELECOM, LAND TRANSPORT, WATER AND ELECTRICITY - ISBN 92-64-02398-4 © OECD 2006 4. KEY FACTORS DRIVING THE FUTURE DEMAND FOR SURFACE TRANSPORT INFRASTRUCTURE influenced (both at the margin and intensively) over several decades, such change is often costly, disruptive, chaotic, politically divisive, etc. and often limited to specific sub-areas of the overall urban form. 45 There is no doubt that more spatially compact and densely populated nations and urban areas support very high volumes of freight and passenger transport within limited, congested urban road networks and extensive public transport systems. Generally, such urban congestion and density mean that transport infrastructure is highly integrated with other urban infrastr