The Transport Department on Wednesday said it doesn't expect new time-varying tolls for the cross-harbour tunnels to bring immediate improvements to traffic flow, as drivers will first need to get used to the changes.

The new fee arrangement comes into effect at 5am on December 17, with higher tolls for private cars and motorcycles to be charged during rush hours.

An assistant commissioner for transport, Leung Sai-ho, said the aim is to encourage motorists to travel outside peak hours and he expects a 10 percent reduction in private cars going through the Cross-Harbour Tunnel during the busiest periods.

"Our main target for the time-varying tolls is not completely making the queue vanish, but instead we are trying to make the queue to an acceptable level so that it will not affect the [non-cross-harbour traffic]," he said.

A chief engineer with the department, Stephen Lee, said the tolls will gradually be ramped up and down either side of peak hours.

"[If not], the toll may change immediately from HK$20 to HK$60 for private cars using the Western Harbour Crossing just in one second. I am afraid that this will be an incentive for drivers to speed up or slow down their vehicles at the toll point. It will create more safety concerns," he said.

From Monday to Saturday, the tolls for private cars using the Western Harbour Crossing will range from HK$20 to HK$60 depending on the time of day, while the fees for motorcycles will range from HK$8 to HK$24.

Cars going through the other two tunnels on these days will be charged between HK$20 and HK$40, and bikes HK$8 to HK$16.

On Sundays and holidays, tolls at the three tunnels will range from HK$20 to HK$25 for cars and HK$8 to HK$10 for motorcycles.