KEY POINTS:
Transit New Zealand contractors will tonight dismantle a giant traffic roundabout in Mt Roskill, ready to open a new road bridge tomorrow above an extension of Auckland's Southwestern Motorway.
They have spent since last Christmas building the $1.5 million Hillsborough Rd bridge, with south-facing ramps to and from the motorway, while almost 40,000 vehicles a day used a 130m-diameter roundabout with an elaborate system of traffic signals to circle barricades around the construction site.
Hillsborough Rd is a main route for commuters travelling between Waitakere and Manukau cities and for traffic heading to and from the airport.
The six-lane bridge will eventually become part of a "full diamond" interchange, with traffic ramps pointing both east and west at the 4km motorway extension being built through Mt Roskill to New Windsor for $195 million.
It is the fifth of six crossings to be built over the motorway route, four of them for general traffic and two others just for cyclists and pedestrians.
Transit regional capital projects manager Sumi Eratne says its completion means contractors, who began building the motorway two-and-a-half years ago, are well past the half-way point towards opening it in mid-2009.
"It's a critical milestone for the project - we have broken the back of it," Mr Eratne said.
But motorists will have to remain patient for some time yet, as the 500m section of Hugh Watt Drive west of the new bridge to Hayr Rd will close permanently tomorrow to allow the motorway to be built in its place.
That will mean a heavier reliance on nearby Carr Rd and Littlejohn St as alternative routes.
Mr Eratne said contractors still had to remove 100,000 cubic metres of earth between Queenstown Rd and Hayr Rd, to give the motorway 6.5m of clearance under the new Hillsborough Rd bridge.
That would reduce the 5 per cent grade of the existing motorway reaching up the hill from Onehunga Bay to just 2 per cent, although traffic would still face a steady climb on the already-completed westbound off-ramp to Hillsborough Rd.
The new 40m bridge, which is supported by an 8m pier, was built out of a tight pit in the middle of the roundabout as everyday traffic movements continued over digging machines and the heads of workers.
Mr Eratne said it was a novel arrangement by which Transit had been able to get the job done without major disruption to traffic.
Two other traffic bridges have already been completed, on May Rd and Dominion Rd, where the motorway's only other interchange with local roads is to be built. So have two elaborate "cable-stay" cycling and pedestrian bridges, between Keith Hay Park and Mt Roskill Grammar School and at Ernie Pinches St at the western end of the motorway.
The only other crossing yet to be completed on the project is the partly-built Hayr Rd bridge further west, although a bridge still has to be constructed to carry the motorway itself across Oakley Creek on the northern boundary of Keith Hay Park.
Transit has yet to widen Maioro St to cope with extra traffic travelling to and from a roundabout which has already been formed at the western end of the motorway, and to extend Sandringham Rd to join the motorway via the roundabout. That will become a main route for traffic heading between the airport and Eden Park for the 2011 Rugby World Cup.
