Traffic on a main route between Auckland and the airport is expected to reduce by more than a quarter once the Waterview Motorway offers faster trips.
A board of inquiry preparing to approve a construction start for the new 4.5km link and widening of the Northwestern Motorway says traffic on Gillies Ave is projected to be 32 per cent lighter in 2016 than it would be without the $1.85 billion project.
It was referring in a draft decision to be confirmed within a fortnight to evidence from Transport Agency traffic consultant Andrew Murray about the effects on local roads once the new motorway completes Auckland's 48km western ring route.
Although the distance between the airport and the central city through an extension of State Highway 20 to the Northwestern Motor way than the Gillie Ave route, the agency expects it to be about five minutes quicker.
A 2005 report to the airport company highlighted a wide variation in trip times on the existing tortuous route, from 16.5 minutes to 47 minutes.
Trip times have become more reliable since a duplicate motorway crossing of Manukau Harbour opened last year, but visitors and commuters still face congestion once they reach local roads.
Mr Murray said daily traffic counts on Gillies Ave were expected to drop from 16,200 in 2006 to 12,100 in 2016 and to 10,700 in 2026.
Other local routes expected to carry less lighter traffic from 2016 include Carrington Rd, with a 25 per cent reduction, Blockhouse Bay Rd (32 per cent), Dominion Rd (22 per cent) and St Lukes Rd (19 per cent).
But motorists will have to put up with delays of up to six minutes during the five-year motorway construction project, which is expected to start before Christmas.