Mr James said subject to the final decision, construction of the expressway could begin in July this year, and was expected to be open by mid- to late-2017.
The expressway would not only provide an improved highway route through the Kapiti district but would also increase safety, reduce travel times and improve journey time reliability between the Kapiti Coast and Wellington, Mr James said.
Transport Minister Gerry Brownlee welcomed today's decision.
"This is a significant milestone towards the progression of this nationally significant road, which would be a key component of the Wellington Northern Corridor Road of National Significance, and provide a much safer, efficient and more resilient transport route north of Wellington.
"April's decision will provide welcome certainty for the people of Kapiti and the thousands of road users who rely on this busy highway lifeline."
Green Party transport spokeswoman Julie Anne Genter said the board's "rubber stamp" came as no surprise, but did not change the fact that the road was a waste of money.
"Can National think of nothing better to do with $630 million of taxpayers' money than spend it on a duplicate highway on a route where traffic volumes are falling?" Ms Genter asked.
"The Kapiti Expressway has a benefit/cost ratio of just 0.2 - that means for every dollar spent, we get just 20 cents of value. What a waste of money."
The expressway had been strongly opposed by the local communities, who did not want to see their "houses bowled and their neighbourhoods split in two" so the four-lane highway could be built, she said.
Six other roads of national significance (from north to south):
* Puhoi to Wellsford;
* Waterview Connection (Auckland);
* Victoria Park Tunnel (Auckland);
* Waikato Expressway;
* Tauranga Eastern Link; and
* Christchurch motorways.
