Hong Kong's private car fleet has passed the 500,000 mark, raising questions about whether curbs are needed - even though the ownership rate is still well below that of other developed cities.
The number could reach almost 540,000 by 2017, the Environment Bureau says.
The prediction brought calls from a green group and an economics professor for fresh consideration of electronic road pricing. But the Automobile Association said there was nothing to worry about.
According to Transport Department data, the number of registered private cars reached 501,021 in March, despite an increase in first registration tax for imported cars in 2011.
Last year, the registered car fleet grew 4.8 per cent, the second-highest rate since 1997. It was also the third consecutive year that the annual rate was more than 4 per cent.
Friends of the Earth director of general affairs Edwin Lau Che-feng said while pollution from private cars was much less than that of diesel buses and trucks, officials should not delay.
"We can start research about what our road capacity is and plan policy measures ahead if required," Lau said, proposing that road charging be considered to discourage car use.
According to department figures, it took the city just seven years to acquire 100,000 more private cars, compared to 14 years leading up to 2007 when the fleet grew from 300,000 to more than 400,000.
But the peak was between 1990 and 1994 when it took just five years to add 100,000 cars.
All imported cars must be registered and a duty paid, and all need a licence.
Ringo Lee Yiu-pui of the Automobile Association, said the present level was acceptable.
"Given the modest increase and present car usage level, the transport infrastructure can still handle 500,000 cars," he said.
Last year, the average car journey speed on Hong Kong Island improved 3.4 per cent to 20.8 km/h, but it fell 1.2 per cent to 23.9 km/h in Kowloon.
Lee warned against any drastic measures such as capping the car fleet size, which he said would attract strong resistance from the public and vested interests.
University of Hong Kong economics professor Timothy Hau Doe-kwong said the rate remained very low compared with other advanced economies. In Singapore, there are 100 cars for 1,000 people. In London, it is 300. In Hong Kong, the figure is 63.
But Hau said road pricing should be considered. "The government had been told to consider road pricing when the vehicle growth rate hit 3 per cent per annum. But this has never happened," he said.