BEIJING – Despite a persistent gray haze, officials said Tuesday an exercise that removed more than 1 million private vehicles a day from Beijing’s gridlocked streets was a success that could mean a clearer sky during next summer’s Olympics.

Humidity and wind conditions kept the pollution from dispersing, but the air during the four-day drill would have been much worse without the vehicle restrictions, said Du Shaozhong, the deputy director of the Chinese capital’s Environmental Protection Bureau.

“The test was successful. These four days the wind speed was slow, while the humidity and temperature were high,” Du told reporters, noting that a gray sky doesn’t necessarily mean pollution is bad.

Air pollution has emerged as a key problem for Beijing as it gears up for the Olympics, to be held Aug. 8-24, 2008. Jammed traffic and the possibility of political protests by critics of the communist regime are also concerns, although venue construction is firmly on schedule.

International Olympic Committee President Jacques Rogge warned during a visit this month that the competition schedule might have to be juggled on days when pollution was particularly bad.

The government’s own statistics showed that the air quality during the test rated among the top 10 worst days of the month so far — and slightly worse that the same period a year ago.

The traffic ban removed 1.3 million private vehicles from the capital’s perpetually gridlocked streets each day. Additional buses and subways were added as residents turned to public transportation, car pools and taxis.

Cars with even-numbered license plates were ordered off roads Friday and Sunday, and vehicles with odd-numbered plates were banned Saturday and Monday. Emergency vehicles, taxis, buses and other public-service vehicles were exempt.

Beijing had an air pollution index of between 93 and 95 during the test days, the city’s environmental protection bureau said on its Web site. According to the State Environmental Protection Agency, an index below 100 indicates excellent or good conditions.

“As the air quality during these four days reached the national standard, it was fit for all activities, including sports,” Du said.

The index hit 116 Tuesday after the test and was 115 on Aug. 16 – the day before the trial began, the Chinese statistics showed.

Officials said a reading of between 101 to 200 indicates slight pollution and people with heart and respiratory conditions are told to avoid exertion and outdoor activities.

Earlier in the month, the pollution index had dipped as low as 42 and generally hovered in the 70s or 80s in the first two weeks of August.