Box 21A Definitions, data availability and country groupings

The most commonly available statistics for transport are vehicle stocks, transport volumes and network length. 'Vehicle stocks' are the number of vehicles of any given transport mode. Car ownership is the number of cars per capita. Transport volume is composed of passenger and freight transport. Passenger transport is usually measured in passenger-kilometres (pass-km) and freight transport in tonne-kilometres (tonne-km).

Compendia of transport statistics are available from several international sources such as the United Nations (UN), the Organisation for Economic Cooperation and Development (OECD), European Conference of Ministers of Transport (ECMT) covering 31 countries, and Eurostat. There are also international bodies such as the UN International Civil Aviation Organisation (ICAO) and the International Road Federation (IRF) which produce regular compendia of transport statistics in certain areas.

However, data are less readily available to show the impacts of transport on the environment. The measurement of emissions from a multitude of mobile sources is inherently more difficult than the measurement of emissions from a limited number of stationary sources such as industrial plants. Many evaluations therefore have to rely on estimates based on emission inventories and estimates about relevant variables (eg, travel speeds, composition of fleets, occupancy rates or load factors). While the availability of such data in Western Europe is patchy, in Central and Eastern European countries they may not exist at all. In addition, many effects are not easily quantified: for example, visual intrusion through transport infrastructure, or the ramifications of transport noise and vibrations for physiological and psychological human well-being. Efforts are being made to devise lists of key indicators to improve representation of the links between transport and environment (eg, OECD, 1993a), but these are hampered by data availability. Recently, a new measure of the link between transport and economic activity ­ gross transport intensity ­ has been suggested based on its counterpart in the energy sector, primary energy intensity (see Figure 21.6).

The country groupings used in this chapter are as in Chapters 19 and 20: EU, EFTA, Western, Central, former USSR, Central and Eastern Europe ­ see Box 19B.