Xia Xiaoyu , who commutes from the suburbs each day in her Mini Cooper, noticed a difference. The 26-year-old self-described environmentalist said she had alternated use of her Mini with another family car.

'I still drove - either in my car or my dad's Land Rover,' she said. 'But the traffic was much better, and air quality probably improved, too. I hope we can have a ban every day.'

But language-school director Timothy Patton, who uses taxis, said the four days were a nightmare.

'Before the ban, there were 100 taxis waiting for one person. Now we have 100 people fighting for one taxi,' he said. 'It is hard to see that the air quality had improved much.'

Taxi driver Zheng Guifan loved the ban.

'Taxi drivers are slaves in Beijing,' he said. 'We work long hours, earn little and are still bullied by bad traffic and ill-tempered passengers.

'The past few days have been our days of liberation. The road conditions improved, and I speeded past countless customers waving their hands in vain. That feeling is good!'

Shi Wenliang defied the ban and drove his car each day to work on the opposite side of the city.

'I condemned the ban,' he said.

'A car is a necessity to those who can afford it. It is so inconvenient without it.'

Greenpeace activist Wen Ning said that although the measure was temporary, the government had to do everything it could to improve air quality for the Olympics.

'Official data suggested the air quality improved,' she said. 'However little, it is worthwhile.'