The light is green for Transmission Gully highway after final approval from the board of inquiry on Friday.
Only a successful High Court appeal can now stop the project, with opponents given 15 working days to appeal the decision, and only on a point of law.
The $930 million highway could be finished as early as 2021. Work is scheduled to start in 2015 on the 27 kilometre four-lane road from Linden to MacKay's Crossing, estimated to shave 10 minutes off peak time travel between Kapiti and Wellington.
NZ Transport Agency state highways manager Rod James said Friday's decision was a ''real milestone'', with the route ''talked about for nearly a century''.
''Every weekday morning and evening we can see that two lanes connecting Wellington to the north is simply not enough to meet our needs.'' Mr James said freight volumes are projected to double in the next two decades, and the route will be ''essential'', and make the region more secure in the event of a natural disaster.
NZTA was ''committed to putting in place extensive safeguards to mitigate the environmental impact of the project'' noted by the board of inquiry.
Kapiti mayor Jenny Rowan was ''delighted'' by the news, and said ''I suspect more residents will be relieved about this decision than not''.
She said Transmission Gully was both nationally and regionally significant.
''It's critical for us now to get a safer and more reliable passageway in and out of the city for those of us who live on the Coast.
She said it was also critical that the first stage of the Kapiti expressway is finished before Transmission Gully, as planned, or the fast moving traffic will ''hit Paraparaumu with nowhere to go''.
The $630 million MacKays to Peka Peka expressway, which will link with the Gully route, is awaiting approval, with work set to start next year and be completed by 2017.
Save Kapiti spokesman Jonathan Gradwell said the Transmission Gully proposal was very different to the MacKays to Peka Peka expressway which he opposed, with the Gully route moving traffic away from the community.
''The proposal on the Kapiti Coast directs traffic further into the community. The arguments that NZTA have used to support Transmission Gully may come back to haunt them with dealing with MacKay's to Peka Peka.''
Greater Wellington Regional Council chair Fran Wilde was delighted by the approval, saying the current road was ''not coping'' with the daily volume of 23,000 vehicles.
''The new Transmission Gully route, which has higher seismic resilience than the present route, will help to future proof our region.''
Transport Minister Gerry Brownlee said work between NZTA and Porirua City Council had helped reduce the expected cost from $1.1b to $930m.
The route will have an interchange connecting with State Highway 58, and another connecting to Porirua City and Linden via the Kenepuru link road.
- Kapiti Observer