A report lodged with the Transport Agency application predicts average daily traffic volumes on State Highway 1 between Puhoi and Warkworth will increase from 17,400 vehicles to 25,000 by 2026 without the dual-carriageway motorway extension with two lanes each way.
Although it acknowledges the new road will induce greater overall traffic volumes, it expects just 14,000 vehicles a day to use the motorway, compared with 14,500 likely to stay on the old route.
But the agency argues the road will be more than what critics deride as a "holiday highway", saying it is needed to cope with a 51 per cent increase in freight movements to or from Northland by 2031, and that it will reduce crashes along what it says is now "primarily a single carriageway, two-lane rural highway with some passing lanes".
It warns that accidents are likely to increase.
The motorway extension - which will form the first stage of a "road of national significance" the Government ultimately wants built to Wellsford once engineers can design their way through tough geological challenges - will include 12 viaducts and bridges through what the agency describes as steep and rugged topography.
The highest viaduct will be 46m above ground level in the Perry Rd sector south of Warkworth, and the longest will be a 520m flyover of the existing SH1 south of Puhoi.
Substantial viaducts will also bypass the existing difficult highway climb over Schedewys Hill.
Northbound traffic will be able to leave the new motorway at Puhoi.
