Key Points:

A new harbour bridge to combat traffic congestion between central Tauranga and Mt Maunganui has reached the halfway mark.

Traffic jams are a daily reality for commuters along the existing bridge - built in 1988 - which has an ideal capacity of 29,000 vehicles a day but is used by 40,000, a quarter of which are heavy goods vehicles.

It is hoped the scheduled completion of the four-way expressway in autumn next year will ease traffic flows between the two sides of the harbour and improve access for Tauranga's busy port.

The harbour link involves building a duplicate bridge over the main Stella Passage harbour channel alongside the existing two-lane harbour bridge, and a viaduct to divert vehicles going to Auckland and Hamilton away from roads into Tauranga's CBD.

The viaduct will also pass over a railway line, and the harbour bridge will include a cycling and pedestrian lane separated from vehicle lanes by a concrete barrier.

Traffic will be shifted on to the new bridge in about eight months so the old one can be upgraded.

A 50m-long, duplicate aerodrome bridge crossing will be built over the secondary channel.

Work on the bridge and viaduct, being built by Fletcher Construction, started in 2007.

Transport Agency regional director Harry Wilson said 15 of 23 bridge segments had been completed. That is more than 250m of its 466m length.

Piling for the bridge should be finished at the end of March, and the bridge should touch down on the Mount causeway in May.

The engineering team includes four people who worked on Tauranga's existing harbour bridge. Project manager Kevin Reid says their knowledge is invaluable because of the engineering challenges presented by the area's poor ground quality.

Piles are driven up to 65m below the harbour floor and 30-40m down on land - because of the silty soil.

The harbour link project was initially going to be funded half by the Government and half by tolls, but is now being completed without tolls.

COMBATING THE TRAFFIC JAMS

* The link is the Bay of Plenty's largest roading project.
* Construction started in 2007 and is scheduled for completion next year.
* During construction, 36,000cu m of concrete will be poured - the equivalent of 6000 truckloads.
* A total of 184 concrete beams up to 37m long and weighing up to 84 tonnes will act as supports.
* The viaduct structure is being built to withstand a one-in-1000-year earthquake and uses 750km of "stress strand" metal cable for reinforcement of the beams.
* The project employs 120-150 engineers and construction workers at any one time.
* The existing harbour bridge, finished in 1988, has an ideal capacity of 29,000 vehicles a day but is used by 40,000, a quarter of which are heavy goods vehicles.
* Together, the existing bridge and the new bridge will have a capacity of 50,000 vehicles.