"Beijing car ban a success, city environmental official says..."

With a growing concern on the idea of Global Warming, there has been much speculation that pollutants from our vehicles in the world's overpopulated streets are a main contributor to the issue. This is shown as a huge environmental issue in Beijing, where there was literally a four-day ban on driving which took about "1.3 million" cars off the road in order to test air quality in a city which is considered to be one of the most polluted cities in the world.
Officials say that after only four days of this test, the air quality in Beijing was improved by 10-20%.
The Deputy of Municipal Environmental Protection Bureau thinks that four days may not have been a long enough time period to permanently improves Beijing's air quality... a place that 17 million people call home.
China will host the 2008 Olympics, which require a certain air quality standard to be deemed "fairly good" enough for the athletes to be able to compete in.
It seems that four days out of a normal 365 day year would not really put a dent in the air quality permanently especially since this is the first year that China has done this experiment. I wonder if even after the Olympics, Beijing will continue this 4day/year ban on cars so that a credible experiment can be conducted to see if a permanent effect even by a small amount can be accomplished over the years?
I would also have to assume that those 4 days will be instantly negated by the number of new vehicles that will be in China for the Olympics in 2008.
If after only four days in one city, air quality was improved by at least 10%, imagine how much effect we would have if even 50 larger cities attempted this experiment over the years.