nks in national/ regional transport networks, and unanticipated changes in freeze-thaw cycles affecting infrastructure integrity and maintenance costs. 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 Country-specific and regional variation The base-year regional/development groupings used in the model for this study reflect cross-country aggregation of national-level data. There are considerable regional/national variations in road/rail networks, vehicle ownership, infrastructure capital value and GDP growth that are built in. 46 The model relies principally on road/rail transport infrastructure capital stock elasticities with respect to GDPpc (with adjustments for: a) increasing road quality over time in route-km costs; and b) convergence in transport infrastructure spending [as a percentage of GDP] towards industrialised levels), and associated road-use elasticities with respect to GDP (and GDPpc for vehicle ownership). While the model growth parameters are insensitive to other factors (e.g. spatial distribution, population densities, demography), such factors are reflected in the base-year disaggregated national data, on which the growth is based. Therefore, the model does incorporate base-year variations in such models, even if it does not allow for growth rate variations (other than GDP growth and GDPPC levels). The underlying growth rate assumptions (e.g. World Bank for GDP growth, UN for population growth) would themselves have recognised, and taken into account the impact of some of these factors. Excluded are any sub-national variations that would affect the distribution of population, economic activity and transport activity within countries. This is the inevitable price of a global focus and aggregation ability, as well as availability of suitable data. Demographic factors There is no doubt that demographic factors (principally the population ageing in most industrialise