Rapidly growing populations and rising incomes will drive an enormous rise in car use and greenhouse gas emissions across Latin America, Africa, the Middle East and Asia by 2050.

But deliberate policy choices about urban planning, road building and the provision of public transport will determine whether the increase is manageable or becomes a nightmare for congestion and climate change.

Car use in the developed economies of North America, Europe, Japan and Australasia appears to have decoupled from economic growth and in some may have slowed down.

But in fast-growing emerging markets, car use will develop in line with GDP unless policymakers provide a strong steer in the opposite direction, according to the OECD's International Transport Forum (ITF).

By 2050, car use in the developed economies is predicted to be around 55 percent higher than in 2010. Economic growth, fuel prices and planning policy are unlikely to affect the outcome much.

In emerging markets, however, car use is projected to rise between 4.0 and 6.5 times. The outcome is critically dependent on oil prices as well as choices about land use, public transport and road building, according to the ITF's "Transport Outlook 2013".

FUTURE CITIES

The UN forecasts that the world's population will rise to around 9 billion by 2050, from 6.8 billion in 2010. The share of those living in cities will climb from 50 percent to 70 percent.

Most of this rural-urban shift will occur in the developing world; of 2.7 billion additional urban dwellers by 2050, more than 90 percent will live in developing countries.

More and more of those city dwellers will be enjoying middle-income status. After adjusting for inflation, the world's income per head is projected to increase from around $10,000 in 2010 to between $20,000 and $28,000 by 2050.

With middle-income status comes a sharp increase in demand for travel to work as well as for leisure and to visit friends and family. So the rising population, coupled with a big increase in the number of middle-income individuals, is set to drive an enormous increase in transport demand.

But car use in future is not something policymakers have to accept passively. It will also be determined actively by the choices they make about land use and transport funding.

"High concentration of population allows urban agglomerations to offer transport alternatives to private vehicles," the ITF explains. "Providing public transport services tends to slow down the increase in car ownership and use as incomes grow."

"Cities hence have the potential to embark on a less private ownership-oriented pathway and rely more on other modes to meet growing mobility demands."

The ITF therefore concludes global passenger trends will be defined by the modes of transport used in urban areas, especially in developing countries.

CAR OWNERSHIP

Energy analysts often assume car use and fuel consumption in emerging markets such as China and India must take the same course as in developed countries like the US, Britain or Germany.

But that ignores huge differences in the level of urbanisation, population density and car use among developed countries and even within them.

There is no single model of car use and fuel consumption in the advanced economies. Even in the US, where car ownership is sometimes portrayed as a matter of personal freedom, enormous variations exist.

More than 56 percent of households in New York City did not own a car in 2012. Roughly a third of households were carless in Washington, Boston, Philadelphia, San Francisco and Baltimore, and about 20-25 percent in Chicago and Detroit.

But car ownership was over 90 percent in Houston, Dallas, Fort Worth, Jacksonville, Phoenix, Austin, Nashville, Charlotte and San Diego, according to a series of research papers published by Michael Sivak at the University of Michigan's Transportation Research Institute

Even in the US, some cities have a strong public transport system while others are dominated by the private car.

Patterns of land use and transportation are partly a matter of public policy as well as private choice.

In India, China and the rest of Asia, the policy impact is smaller, but it could still affect car use by a factor of around 0.5. Given how large these economies are, and how many potential future drivers they have, the impact on fuel consumption and emissions is enormous.

"The scope for relying on public transport to meet mobility needs is broader in cities than elsewhere," the ITF concludes. "Urbanisation can result in a lower share of cars meeting transport demand even if urban incomes are higher. However, realising this potential requires supporting policy."

Pessimists often assume higher fuel demand and emissions are essentially unavoidable if developing economies are to converge with per-capita incomes in the developed world. But the ITF's analysis shows future car use and fuel consumption levels are in big part a matter of choice, too. - Reuters

* John Kemp is a Reuters market analyst. The views expressed are his own