The Expressway Authority of Thailand (Exat) has proposed a tunnel to replace a controversial section of the N1 Expressway development.

Exat governor Surachet Laophulsuk said on Monday the agency had considered several aspects and found the underpass the best idea.

The 6.7km underpass, which will be the country's first expressway tunnel, will run along Ngarmwongwan Road via Phongphet Intersection, Bang Khen Intersection, and Kaset Intersection to Prasertmanukit Road before connecting with the N2 Expressway.

Mr Surachet said the environmental impact assessment will end in May next year for submission to the cabinet for approval.

Construction is likely to take about five years, and in the first year of opening, traffic volume is estimated at 70,000 vehicles per day, half of the route's total capacity.

He expressed confidence that the underpass would be approved by those who had environmental concerns about the previous design.

Mr Surachet said the northern part of the three-phase expressway development is divided into two stages.

The first stage, N2, runs from Prasertmanukit Road to the eastern Outer Ring Road, with a length of 11.3 km and an investment cost of 16.9 billion baht.

For the second stage, N1, Exat has conducted the feasibility study of alternative routes to replace the controversial section and come up with the underpass following discussions with Kasetsart University and communities along the planned route.