The Transport Department on Friday appealed to motorists to drive safely at a new toll-free tunnel that connects Tseung Kwan O and Lam Tin, saying heavy traffic is expected when it opens.

The 3.8-kilometre connection and the adjacent Cross Bay Link, a 1.8-kilometre bridge, will open at 8am on Sunday.

Starting Monday, five new franchised bus routes will also be using the new tunnel for peak-hour services between Tseung Kwan O and Tai Po, Tsuen Wan, Sham Shui Po, Cheung Sha Wan and Tsim Sha Tsui.

Chief transport officer Daniel Hue called on motorists to drive safely, adding that the Transport Department will be monitoring the traffic at the new tunnel closely.

"The traffic on the new roads is expected to be busy in the early stage after the commissioning of the new Tseung Kwan O-Lam Tin Tunnel. Motorists to and from Tseung Kwan O are advised to choose their route, including the existing Tseung Kwan O Tunnel, based on the actual traffic situation and their destination," he said.

"The Transport Department has already erected directional signs to facilitate the motorists. The motorists are also reminded to reduce the speed and drive safely at the new roundabout and junction of the new roads."

Hue said the new route should divert some of the traffic from the old Tseung Kwan O Tunnel, which will also become toll-free from 12am on Sunday.

He said some traffic lanes of the old tunnel will be closed from Saturday night for modification works to take place.