13 25 These estimates are larger than earlier ones. If estimated vehicle ownership saturation levels change over time, they are of little use for forecasting. In fact, that seems to be the case as there is little direct evidence that saturation levels are stationary or that they have a straight forward behavioral interpretation. The income elasticity of motor vehicle ownership may decline as incomes rise. Estimates allowing income elasticities to vary with country income produce declining income elasticities but do not have greater explanatory power than constant elasticity specifications.26

# **5. Road Provision**

What are the determinants of road provision, or the increase in road networks, at the national and city level?<sup>2 7</sup> Unlike motor vehicles, roads are not usually privately provided, and road provision may not be strongly conditioned by economic considerations. Nonetheless, knowledge about the recent trends in road provision and its relation to income, population, and settlement patterns is a useful guide to future road provision and an important input to transport policy making. Although the impact of economic development on roadway networks has long been a subject of descriptive studies for several countries, cross-country empirical studies are recent.<sup>2 8</sup>

# _5.1 National road networks_

The few available studies of road provision across countries are based on data that provide information on road length but not on road width or numbers of lanes. Data available for paved roads and total roads are reported by the countries and generally include urban roads, but country-level definitions sometimes vary with respect to coverage and technical classification. Data on paved roads are more comparable across countries 25 Two of the previous estimates (or assumptions) were provided by Tanner, "Forecasts of Future Number of Vehicles;" and Mogridge, "The Prediction of Car Ownership." Tanner predicted a saturation level of 450 cars, and Mogridge, 660, per thousand persons for the UK.

26 Ingram and Liu, "Vehicles, Roads, and Road Use." 27 This section draws heavily from Ingram and Liu, "Motorization and the Provision of Roads," and "Vehicles, Roads, and Road Use." 28 Esra Bennathan, Julia Fraser, and Louis Thompson, "What Determines Demand for Freight Transport?" Policy Research Working Paper 998 (Washington, D.C.: World Bank, 1992); David Canning, "A Database of World Infrastructure Stocks 1950-1995" Policy Research Working Paper 1929 (Washington, D.C.: World Bank, 1998); and Ingram and Liu, "Motorization and the Provision of Roads," and "Vehicles, Roads, and Road Use." Bennathan and others used data from 36 countries to analyze the relation of domestic rail and road freight transport demand (in ton-kilometers) to country income and land area variables. Canning analyzed several infrastructure stocks--including roads, telephones, and electric generating capacity--using a panel data set containing from 95 to 145 countries. Ingram and Liu analyzed the regularities of provision of roadway length using data from 50 countries and 35 to 37 cities spanning a wide range of income levels; their findings are summarized in this section.