Beijing is pulling more than a million cars from the streets to test the effect on air quality and traffic flow. The four-day experiment is part of a search for ways to improve the Chinese capital's notorious air pollution and horrific traffic jams during next year's Olympic games. Daniel Schearf reports from Beijing.

For four days, most vehicles in Beijing will only be allowed to take to the road on alternate days - odd-numbered license plates one day, even numbers the next.

China's official Xinhua news agency says the temporary measure is expected to remove more than a third of Beijing's three million-plus vehicles from city streets.

If the test leads to significant improvements in air quality and traffic flow, Beijing may use a similar method during the Olympic games next year, when 2.5 million visitors are expected to put added strains on the city's already clogged transportation systems.

Wen Bo is based in Beijing-based for Pacific Environment, a San-Francisco environmental group. He says limiting cars might affect traffic, but is not likely to have a big impact on Beijing's air quality. He says the city is surrounded by too much polluting industry for temporary measures to make much of a difference.

"Lots of factories have been relocated out of Beijing but they are still surrounding the city. So, they have to deal with the entire region. It's not just one city," he said. "You cannot just have a pocket of clean air and a sea of polluted air all around it. By reducing car numbers on the street, you're only dealing with one section of the problem."

Olympic teams are worried that the heavy air pollution may affect their athletes' health during the games.

The International Olympic Committee president said some endurance events might have to be postponed if Beijing's air quality is too poor.

The Chinese government is counting on the games to highlight the country's emergence as a world economic power. If the games are tainted by poor air, or if athletes can't get to their venues because they are trapped in traffic, it would be a major embarrassment for the hosts.

Officials say particulate matter, a major element of air pollution, has been reduced in recent years, but the rapidly increasing number of cars on the capital's roads poses a new challenge to air quality.

As the economy has raced ahead, the city's well-to-do have bought cars to show off their new wealth, adding a thousand new vehicles to Beijing streets each day.

Before the Olympic Games begin in August 2008, the city government plans to more than double the number of subway lines and put more buses on the road to encourage the use of public transportation. Half of Beijing's more than 20,000 buses already run on clean natural gas, and the other half are due to be converted by the end of this year.

But officials are concerned these steps may not be enough. Draconian measures, like ordering vehicles off the streets, may be necessary.