Slow-down ordered on shrinking viaduct

The new Newmarket Viaduct is gradually taking shape. Photo / Glenn Jeffrey

Traffic on one of New Zealand's busiest motorway stretches, Auckland's Newmarket Viaduct, will face tougher speed restrictions from Monday as a giant replacement structure sprouts up next to it.

The Transport Agency will drop the 100km/h limit to 70km/h on the southbound lanes and to 80km/h in the opposite direction, to reduce the risk of crashes if drivers become distracted by the construction of a $215 million pair of replacement viaducts.

Drivers will also have to cope with with narrower traffic lanes.

The lanes will be shrunk from 3.5m to 3.1m in an overnight closure of the southbound carriageway on Sunday, making room for a screen to be constructed along the 690m northern lip of the structure.

A merging lane for southbound traffic flowing past the Gillies Ave motorway off-ramp to the viaduct will be shortened.

The screen will reduce visual distractions from the construction project and create a safety buffer for workers erecting a new four-lane motorway deck 13m to the north.

Three of the 24 columns needed to support the replacement viaduct have been largely completed, to heights of up to 20m, and the Transport Agency's construction alliance expects to start erecting deck spans between these by the end of this month.

Once a platform is formed by the initial deck spans, a 140m gantry will be erected high above the traffic to manoeuvre the rest of the new southbound structure into place.

Prefabricated sections for the 700-tonne gantry have been enlarged and strengthened since they were used to build the Waiwera River viaduct on the Northern Gateway toll road, which opened in January.

Transport Agency regional highways manager Tommy Parker said yesterday the restrictions would have little effect on peak-hour traffic, which travelled at an average speed of only about 30km/h over the viaduct.

But they would be important at off-peak times, to reduce the risk of vehicles spinning out of control in a more confined motorway corridor.

"We are making these changes for the safety of drivers as they travel through a live construction zone," he said.

They would help the agency to keep all three southbound lanes open "to carry out a vital replacement of a busy stretch of road without bringing Auckland to a halt".

Mr Parker said the police had undertaken to enforce the new speed limits but he appealed to drivers to co-operate voluntarily to ensure the three lanes could remain open.

"If we find that drivers aren't slowing down or there is a high risk, our only option would be to close the outside lane and that would have a huge effect on capacity, so we really want to avoid that."

The viaduct carries more than 160,000 vehicles a day, and the agency hopes to have the replacement southbound structure available in time for the 2011 Rugby World Cup.

That will be followed by the demolition of the southbound carriageway, to make way for a replacement northbound viaduct by early 2012.

A new auxiliary motorway lane to be built south of the viaduct for about $18 million will increase capacity to four lanes as far as Greenlane by 2011, but the northbound structure will be limited to three lanes to prevent any imbalance with the tight vehicle space available through Spaghetti Junction.

Although that will provide no more capacity than the existing structure, the replacement viaducts have been designed to have five times more seismic resistance than the present structure - able to withstand a once-in-2500-years earthquake.