BEIJING — Authorities Sunday ordered half of Beijing's 3.3 million vehicles off city streets for a two-month period in an experiment designed to remove a smoggy halo over the capital and ease traffic jams before the Summer Olympics.
Poor air quality is a major concern for the Aug. 8-24 Summer Games, already causing a few high-profile athletes to pull out.
Patrolling tow trucks impounded vehicles that violated rules that restrict cars to alternate days based on whether license plates end in odd or even numbers.
Authorities forecast that the sweeping traffic restrictions, along with a major expansion of the capital's subway system that began over the weekend and measures to shut down polluting factories, would help clear smog over Beijing in time for the Games. Some foreign experts voiced doubts, saying air pollution is too intense to clear so easily.
It marked the second time in less than a year that Beijing restricted traffic. In a four-day test last August, pollution experts noted a slight improvement in air quality.
Kenneth Rahn, a retired atmospheric chemist at the University of Rhode Island who has analyzed air pollution data from Beijing over the past five years, said weather factors, such as winds bringing clean air from the Mongolian steppes, may make more of a difference to air quality during the Olympics than the traffic restrictions.
"Last summer's experiment gave undetectable reductions in pollutants overall," Rahn said in an email, adding that the traffic reduction might be ineffective but harmless to attempt.
"On the philosophy that every little bit helps, this effort is probably worth doing," he said.
The restrictions began on a lightly hazy day with distinct patches of blue in the sky. Looking down the capital's main Boulevard of Everlasting Peace, the Western Hills loomed visible, a rare visual treat.
Beijing residents have gone car-crazy this decade. About 1,200 new vehicles pile onto the city's streets each day, slowing transit on the five concentric ring roads around the capital to a stop-and-start crawl at peak hours.
Roads on Sunday, though, were remarkably free of traffic.
"You can see there on the third ring road," said travel tour organizer Lang Zhijie, sitting in his shaded vehicle. "Normally, it's jammed. Today, traffic moves very fast."
A $3.2 billion expansion of the city's subway system, inaugurated over the weekend, lengthened its routes to 125 miles, a 40 percent jump. One of the three new lines carries passengers from the airport to a downtown hub in a brisk 20 minutes.
The three new subway lines will carry an additional 850,000 passengers a day, the state Xinhua news agency said.
The car restrictions will require some four million additional passenger journeys each day to be made by public bus, subway, bicycle or taxi, burdening the public transit system. Xinhua quoted Zhou Zhengyu, a city transportation official, as saying 2,000 additional public buses would run during the two-month period.
The city's 66,000 taxis are exempt from the odd-even restriction.
Beijing has taken other major steps to reduce pollution in recent years, including ordering smelters, brick kilns, cement, petrochemical and steel plants to cut emissions or move from the city entirely. Coal-fired power plants also have installed cleaner technology.
The measures have utterly reversed the course once set by Mao Zedong, the founder of Communist China in 1949, who sought to end Beijing's vaunted role as an imperial city and turn it into a workers' haven of factories, steel mills and smokestacks.
In a sign of the change, authorities led reporters on a tour last week of the now-quiet blast furnaces and mills of the Capital Iron and Steel Group, a huge steel maker known as Shougang that for much of the Twentieth Century was the city's worst polluter. The plant's operations have largely been relocated out of Beijing to nearby Hebei province.
Shougang Group President Zhu Jimin said production this year is only 27 percent of what it was last year, limiting the black soot emissions that once coated Beijing.
Three of four of the company's blast furnaces have been shut down.
Rahn, the American atmospheric chemist, said such measures help in the battle against air pollution but that contamination affects a vast area of northern China.
"It's one thing to take steps to try to clean up a big city, but unless they also clean up the surrounding provinces, it's going to have a minor effect," Rahn said in a statement sent out by his university.
(McClatchy Newspapers special correspondent Hua Li contributed.)