New Zealand Road history: Ngauranga Interchange

It's 1982, and the National Roads Board (an arm of the Ministry of Works and Development (MOWD)) had approved the Ngauranga Interchange. This interchange which opened in 1984, finally linked the Wellington Urban Motorway to SH1 in Ngauranga Gorge, enabling the vastly underutilised capacity of the Urban Motorway to be used, and relieving chronic congestion at the Ngauranga intersection between the Hutt Road and Ngauranga Gorge Road/Centennial Highway (as Ngauranga Gorge's state highway was known). Of course having three lanes each way in Ngauranga Gorge coming to a halt at a traffic light controlled intersection saw significant traffic jams in the AM peak, and likewise in the PM peak traffic backed up south along the Hutt Road. Indeed before the Ngauranga Interchange, the Hutt Road had along much of its length from Ngauranga to Aotea Quay (as SH1) had a tidal flow lane that operated at peak times in the direction of peak flow. One limitation of the Interchange, which was due to the limitations of the Urban Motorway, is that Cook Strait Rail Ferry (now Interislander) traffic, including heavy vehicles, would have to continue to exit at Ngauranga and use the Hutt Road and Aotea Quay ramps to access the ferry terminal (this is about to be addressed with construction of a new roundabout on Aotea Quay to enable traffic to exit the Urban Motorway at Aotea Quay and reverse to access the Interislander terminal).

This was one of the last major road projects in Wellington carried out by the MOWD before the National Roads Board and Ministry of Works were abolished, replaced by Transit New Zealand (as the combined state highway manager and land transport funding agency) and Works Corporation (which was subsequently privatised), which itself was subject to competition in highway construction as the sector was opened up to mandatory outsourcing.

The Ngauranga Interchange demonstrated some of the best construction and planning the MOWD did in its latter years, and has stood the test of time. It permanently relieved the congestion at the Ngauranga intersection, although today the key congestion comes from traffic between the Hutt and the Northern Suburbs/Porirua. The Interchange itself was originally built with one lane dropping/feeding to and from the Urban Motorway, and the second lane breaking/merging. More recently a fourth lane was added by narrowing the shoulder, northbound from Aotea Quay to Ngauranga, relieving northbound congestion, and the southbound merge extended to enable smoother merging.

The legacy of the Ngauranga Interchange includes several other impacts:

SH1 status was removed from the Hutt Road and its Aotea Quay ramps and shifted to SH1 (but remained on the Aotea Quay motorway ramps and Aotea, Waterloo, Customhouse and Jervois Quays terminating at Taranaki St until the 1990s).
It enabled the Hutt Road/Kaiwharawhara and Ngauranga areas to grow their commercial and retail activities, as the removal of SH1 from that road enabled much easier access to those locations.
It relieved the waterfront route along Aotea/Waterloo/Customhouse/Jervois Quays, as SH1 traffic was not required to use that route to get to Te Aro or other suburbs.
It saw the limitations of the end of the Urban Motorway at Ghuznee/Vivian St (with one Terrace Tunnel) become obvious, as congestion there and through Te Aro became a regular occurrence.
It gave Wellington one of the most dramatic and beautiful entrances by highway into a capital city anywhere in the world.

Below is a leaflet published by the Government Printer (another government agency privatised subsequently) promoting the Ngauranga Interchange before it was built (apologies for the writing/drawing in parts from school age me)