Plans for a one-year trial aimed at cutting congestion in the Cross-Harbour Tunnel will be published for public consultation "very soon", a source said.
It is intended to raise tolls in the tunnel, from Hung Hom to Causeway Bay, and reduce the cost of using the Eastern Harbour Crossing, from Quarry Bay to Cha Kwo Ling.
A consultant's report in 2010 said such an arrangement could divert 4,300 of the 120,000 vehicles passing through the government-owned Cross-Harbour Tunnel each day to the eastern crossing, which is used by 69,000 vehicles per day.
The study suggested raising the Cross-Harbour Tunnel toll for private cars from HK$20 to HK$25 and cutting the cost at the eastern tunnel from HK$25 to HK$20. But the source said the government would present "a few plans" for the public to discuss, with the fare adjustment and the types of vehicles targeted yet to be finalised.
The government would review the effectiveness of the measures after a year, he said.
Chief Executive Leung Chun-ying said in his policy address he would consult the public on a plan to adjust tunnel tolls in the first half of the year.
He said the government may compensate the owner of the eastern crossing, the New Hong Kong Tunnel Company, for any loss in revenue.
But the source said there were no plans for the trial to include the third harbour tunnel - the privately run Western Harbour Crossing that runs from West Kowloon to Sai Ying Pun - which charges cars a toll of HK$55 per crossing and has far less traffic.
He said the connecting roads would struggle to accommodate more traffic before the Central-Wan Chai bypass opens in 2017.
While some lawmakers are opposed to raising the cost of using the Cross-Harbour Tunnel, the source said this was not an option if traffic was to be reduced.
"If tolls remain the same, it will stay as congested as it is now," he said