Professor Lau said the government should be able to release precise calculations of the expected reductions in emissions of sulfur dioxide, carbon monoxide, nitrogen oxides and respirable suspended particulates. 'I can't judge whether it is a feel-good policy or an effective tool when these questions are still unanswered,' he said.

Lo Kok-keung, of Polytechnic University's mechanical engineering department, said the ban could improve roadside air quality but the information released lacked details.

'Perhaps the government is still unable to come up with a rough estimate and that's why it hasn't put it in the consultation paper,' Mr Lo said.

He estimated the pollutant emissions in busy districts such as Causeway Bay and Mong Kok could drop by 10 per cent if a ban was implemented.

In the consultation paper, the government compares the emissions of vehicles while running and when stationary with engines idling.

An idling diesel-engined public light bus emits about half the pollutants of one in motion. Cars running on petrol emit almost as much carbon monoxide and hydrocarbons when their engines are idling as when they are in motion.

The government admits emissions from idling engines are 'small in quantity as compared to emissions from the entire vehicle fleet', but says they cause air pollution and noise nuisance to nearby pedestrian and shops.

There are 550,000 vehicles on Hong Kong's roads.

Fung Man-keung, lecturer in automotive engineering at the Hong Kong Institute of Vocational Education, said engines must be switched off for at least three minutes for there to be any effective reduction in the pollutants emitted.

'According to overseas studies, if a driver keeps turning an engine on and off every two minutes, the time the engine is turned off has no significant impact on pollution and the action merely speeds up the deterioration of engine components,' Mr Fung said.

Au-yeung Ming, a spokesman for the Motor Transport Workers General Union, which opposes a ban, said vehicle starter motors would wear out much more quickly if the proposal became law.

He estimates the lifespan of a starter motor would be shortened by two years, to five years.

'If we keep switching the engine on and off while waiting in a long queue, we will only emit more pollutants,' Mr Au-Yeung said.

Pollution hazard

Vehicles are the second-largest source of air pollution in Hong Kong, contributing to 25% of respirable suspended particulates, 25% of nitrogen oxides and 15% of carbon dioxide

While idling engines do not contribute as much as those in motion, the white paper recognises the problems they cause at ground level in congested areas, and the growing number of complaints

Exemptions from the ban include:

Vehicles which stop at the roadside to let passengers board or alight; the first two taxis or public light buses at a stand; vehicles remaining motionless because of traffic conditions such as traffic congestion, security vehicles