The Government's $12 billion roads of national significance programme is upgrading routes used by as little as 4% of the country's traffic.
The Green Party wants the programme scaled back and says its research shows the routes carry only 400,000 of our 11 million daily vehicle trips.
Green transport spokeswoman Julie Anne Genter said the benefits did not justify the spending.
"The spending is disproportionate to the number of trips. We are spending money on projects you are unlikely to drive on. It's an enormous amount for a few roads that have almost no traffic growth."
Supporters say the improved network will increase traffic flow and lead to a lift in business productivity, but Genter said the benefits would be negligible.
Transport Association chief executive Geoff Dangerfield said highways played a significant role in trade, competitiveness and economic growth.
About $50b worth of goods entered the country each year, and 92% of all freight, by weight, was moved by road.
The seven programme routes were linked to economic prosperity.
"These roads are linked to our five largest economic activity centres, Auckland, Hamilton, Tauranga, Wellington and Christchurch. Moving people and freight more safely and efficiently represents a critical investment. The programme improves access to our ports and markets, and puts infrastructure in place that will encourage growth."
Dangerfield said the project would give gains "significantly beyond the costs", and they would accrue beyond the 30-year time period the agency used to assess the benefits.
Transport Minister Gerry Brownlee recently told Parliament he had no doubt the project would bring economic, social and safety benefits.