Transmission Gully could make the Ngauranga Gorge bottle neck worse if the proposed Petone-Grenada link road is axed.
The impact on Wellington traffic after the Gully road is built was based on the link road going ahead and easing congestion.
Greater Wellington regional council senior transport planner Natasha Hayes confirmed, under cross examination today, that no modelling had been done on the impact of congestion if the link road wasn't built.
"[From a] policy point of view this project is one of many projects that will provide better access to the CBD and airport," Ms Hayes said.
The 6.4km Petone-Grenada road is part of a proposed $250 million package and is forecast to carry 25,000 vehicles a day by 2026.
The council's lawyer Kerry Anderson said the council partially supported the inland Gully route, in her opening submission to the board of inquiry today.
The board of inquiry will decide whether to give the $1 billion project the green light in June.
If approved, construction could start in 2015 and be completed in six years.
The 27km inland route would carve through farm land and dozens of streams linking Linden to McKays Crossing.
Ms Anderson told the board issues relating to access through Battle Hill Forest Farm had been resolved through an agreement with the New Zealand Transport Agency.
However, it would not enter into an agreement regarding bulk water supply, rather it sought a condition from the board should the project's consents be granted.
NZTA, Porirua City Council and Transpower lodged 5000 pages of resource consents and notices of requirement with the board last year.
Planning issues including a separate report on vegetation restoration was also outstanding, Ms Anderson said.