BEIJING — With the Olympics less than three weeks away, Beijing began restricting car use and limiting factory emissions on Sunday in a final drastic effort to clear its smog-choked skies.

Under the two-month plan, half of the capital's 3.3 million cars will be removed from city streets on alternate days, depending on whether the license plate ends in an odd or even number.

Skies were relatively clear on Sunday after some morning haze, and traffic was light for a weekend, flowing smoothly on highways and city streets. But the real test will come when the work week begins.

"Things are fine today," a taxi driver who gave only her surname, Li, said as she sailed through normally traffic-snarled intersections. "But tomorrow, it may be different as people go to work."

In addition to the traffic plan, chemical plants, power stations and foundries had to cut emissions by 30 percent beginning Sunday. Dust-spewing construction in the capital was to stop entirely.

While the government has said it hopes to reduce vehicle emissions, one of Beijing's chief sources of pollution, it is unclear how the effectiveness of the plan will be gauged. The government has not made public a specific target for emissions levels or said how it will measure air quality.

Despite architecturally adventurous venues and $40 billion spent on improving infrastructure, China's greatest challenge has been keeping the city's air clean for the world's greatest athletes participating in the Aug. 8-24 games. Beijing's skyline is normally shrouded with a thick gray haze.

Already, many competitors are choosing to train away from Beijing, and International Olympic Committee President Jacques Rogge has said outdoor endurance events lasting more than an hour will be postponed if air quality if poor.

The world's greatest distance runner, Haile Gebrselassie of Ethiopia, has decided not to run the marathon event because the city's pollution irritates his breathing.

Some 300,000 heavily polluting vehicles — aging industrial trucks, many of which operate only at night — were banned beginning July 1.

To further ease the gridlock, employers have been asked to stagger work schedules and public institutions will open an hour later than normal. And those driving on the wrong days will be fined $14, a pricey penalty for many in Beijing.

The government has also beefed up public transportation options for the estimated 4 million extra people who will be off the roads because of the traffic plan, the official Xinhua News Agency said.

The city is scheduled to add up to 3,000 more buses by the time the Olympics start, raising the daily capacity for passengers from 12.5 million to 15 million, it said.

A rare 95 percent of buses reached their stops on time on Sunday because the roads were less congested, Xinhua cited Yao Zhenping, assistant to the general manager of the Beijing Public Transport Holdings Group, as saying.

Two new subway lines and an airport rail link opened on Saturday, with the projected number of passengers on all three routes expected to carry 1.1 million people daily during the Olympics.

"It's much faster than a taxi, so it's very good," said Ola Tunamder, who arrived Sunday from Helsinki, Finland, and took the airport train into the city.

And on July 25, special Olympic traffic lanes will begin operating and will stay in place until Sept. 25. The city will set aside 165 miles of roadway on which certified Olympic vehicles will be allowed to move from hotels, Olympic venues and the Athletes' Village. The average speed is expected to be 35 mph.

Experts say the pollution-curbing experiment could still go wrong.

Unpredictable winds could blow pollution into Beijing despite factory shutdowns in the city and five surrounding provinces. Or it could go the other way, with August generally being the month with little wind, potentially allowing pollution to build up.

Also Sunday, Xinhua said that China's civil aviation authorities had begun implementing air traffic control measures to deal with potential emergencies.

Citing Su Langen, an aviation official, the report said that authorities "will maintain closer monitoring of air traffic" but did not give any details.

"They will respond quickly to emergency situations such as terror attacks, hijacking of civil aircraft, and unauthorized entry of no-fly zone," Xinhua said.