Legislators on Tuesday cast doubts on a government proposal to change tunnel tolls in a bid to ease congestion, saying they do not see figures or data to back it up.

The government wants to reduce the tolls for private cars using the Western Harbour Crossing to HK$60 when it takes back control of the tunnel next summer, and raise the tolls at the Cross-Harbour Tunnel and the Eastern Harbour Crossing to HK$30.

At a Legco panel meeting, lawmakers including the DAB's Chan Hok-fung, said the government did not provide estimates to help them gauge the effectiveness of the plan.

"There is no data in the document. I don't know how much [traffic] volume will be reduced after the toll adjustment, and I don't know how much time can be saved," he said.

Joephy Chan from the Federation of Trade Unions said even when the adjustments are made drivers may prefer the Cross-Harbour Tunnel to the Western Harbour Crossing, as the latter will still cost twice as much to use.

In response, the Secretary for Transport and Logistics, Lam Sai-hung, said he believed the plan will distribute traffic more evenly, when the price difference between the harbour tunnels is narrowed from HK$55 to HK$30 next year.

When asked why different toll levels couldn't be brought in at different times of the day to ease congestion, Lam reiterated that the authorities plan to do so, but only after monitoring traffic data for a year after the tolls are adjusted.

"We are still in the process of returning to normality, and the volume of traffic is not stable yet. We need some time to look at the traffic flow. After it has stabilised... we can implement [different toll levels at different times of the day]," he said.

The DAB's Lau Kwok-fan raised concerns about the government's plan to introduce a toll adjustment mechanism that would allow it to "fine-tune" tunnel charges without the need for legislative amendments. He asked whether that would mean the government could raise the tolls at anytime in future.

The Commissioner for Transport, Rosanna Law, said officials would only propose small increments, like HK$2 or HK$3, if needs be, and there would definitely be "checks and balances".