Michael Bristow reports from a Chinese motorway
Traffic restrictions have been re-introduced in China's capital Beijing, in an attempt to bring back the clear skies seen during the Olympics.
Each car must spend one day a week off the road, in a scheme based on registration numbers.
Beijing residents miss the clean air and quiet streets they enjoyed throughout the Games.
But according to correspondents, many people have complained that the new car scheme is too complicated.
The new rules are expected to take some 800,000 cars off the road every day, according to the Beijing Municipal Committee of Communications.
"It's expected to reduce Beijing's average road traffic flow by 6.5%," a committee official told the state news agency Xinhua.
During periods of exceptionally heavy pollution, the restrictions will be increased so that half of Beijing's 3.4 million cars will be taken off the roads, state media reports.
The new rules came in to effect on Monday morning - anyone with a licence plate ending in one or six could not drive into the city.
On Tuesday it will be the turn of people with number plates ending in 2 and 7.
The new restrictions will be implemented on a trial basis for six months until April.
A similar scheme operated during the Olympics, when factories around the city were also closed to reduce air pollution.
The measures transformed the city's filthy skies, and according to a BBC correspondent in Beijing, Quentin Sommerville, it was the cleanest August that Beijing had seen in 10 years.
But the authorities face a mounting battle to reduce car use in Beijing - 1,000 new vehicles join the crowded roads every single day.