By Michael Bristow
BBC News, Beijing
Bicycles once used to rule the roads in central Beijing
Beijing is expected to pass the four million vehicle mark sometime during the next few days.
Traffic officials said there were 3.99 million vehicles on the road by last Sunday. About 10,000 new ones are added each week.
Officials have spent billions of dollars on public transport over the past few years.
But a city that used to be famous for its bicycles now has congested streets just like many other modern cities.
Beijing Traffic Management Bureau made the announcement that the city would soon pass the four million car mark, and it also said the city had nearly six million drivers, out of a population of about 17m.
Beijing's city government acknowledges that the increasing number of cars on the road is a problem.
"The emergence of a concentrated number of new cars and drivers continues to increase the pressure on road transport," said a statement from the city's traffic bureau.
"This has caused our city's roads to become over-saturated."
There were just 2,300 vehicles on Beijing's roads when the Communists took over in 1949, according to reports.
It took another 48 years to get to one million cars - but only two years to go from three to four million.
Bicycles started disappearing from the city's streets when economic reforms gave ordinary people the money to buy cars. In addition, many other cars are owned by government departments and work units.
But roads quickly became clogged up, forcing the government to limit the amount of time cars can drive. Air pollution has also been a problem.
To solve these issues, the authorities have invested heavily in public transport over recent years, a programme that was accelerated when Beijing was given the 2008 Olympic Games.
But Professor Ou Guoli, from Beijing Jiaotong University, says more needs to be done.
"We need other government policies and measures to reduce the amount of traffic on the roads," he said.
Prof Ou said city centre parking should be made more expensive and there needs to be more "park and ride" facilities.
There should also be policies to encourage people to get back on their bikes, he said.