Drivers are being urged to take public transport and leave their cars at home once a month to reduce carbon emissions and tackle climate change.
In the campaign Take a Brake Low Carbon Action, companies and owners are asked to commit to cutting at least one driving day a month and to improve driving habits to reduce fuel consumption.
The campaign was initiated by Standard Chartered Bank, which invited green groups Friends of the Earth, Green Power and WWF to educate the public and lobby companies.
At least 45 companies and more than 400,000 private car owners are expected to join the campaign, which would reduce the city's carbon emissions by 660 tonnes in a year.
But so far the government has not given a commitment to joining.
The calculation assumes a driver travels 20 kilometres a day in a private car with a cylinder capacity of 1,500cc to 2,000cc, which produces 150 grams of carbon dioxide for each kilometre.
Friends of the Earth director Edwin Lau Che-feng said nine companies had already joined - CLP Power, Hong Kong Electric, Sino Group, Wheelock and Co, Watsons Water, Hong Kong International Airport, Crown Worldwide Group, Dow Chemical Pacific and Hong Kong and China Gas Co. Apart from reducing driving days, the companies would aim to cut fuel consumption of their fleets by at least 2 per cent.