What are the key factors?

Income
In most countries there appears to be a close association between GDP per capita and passenger km travelled (WBSCD, 2001). As incomes increase, people switch to faster modes of transport, which also require more energy per passenger kilometre. An interesting point is that although distance travelled on average increase with income, the amount of time spent travelling does not (WBSCD, 2001). The availability of faster modes of transport at lower cost brings them within the reach of an increasing number of people.

Increases in car ownership are related to increasing income. In middle-income countries (with average annual incomes between about $5,000 and $15,000) for every 10% increase income tends to result in a 10% increase in vehicle ownership (Pew, 2002), although this varies considerably depending other factors such as public policy and local market conditions.

Car ownership is however, still far lower in developing countries, with 22 cars per 1000 inhabitants in Shanghai compared with 210 in Tokyo and 340 in London. There is therefore the potential for a huge increase in car ownership and hence emissions.