The Office of Transport and Traffic Policy and Planning (OTP) will forward its feasibility study of the underground road project linking Bangkok with Samut Prakan to the Ministry of Transport by the end of this year.
The study explores the possibility of building an underground expressway from Narathiwat Ratchanakharin Road to Samut Prakan's Samrong district.
"The initial result suggests it's possible to build the country's first underground road.
It should help ease congestion in inner city areas," said OTP deputy director Chayatham Promsorn.
He said the tunnel will be operated as an expressway, meaning motorists will have to pay to use it.
According to Mr Chayatham, the project is part of the OTP's attempts to solve traffic woes in Bangkok and surrounding provinces as instructed by Prime Minister Prayut Chan-o-cha.
He said Gen Prayut suggested the tunnel as one of the solutions as the capital is running out of surface space to build roads or elevated roads.
A source at the OTP said the underpass will be about 9 kilometres long and may be split into two levels for private cars and public transportation.
If completed, it should alleviate congestion in the Rama IV Road, Sathon, Silom and Bang Rak areas.
Construction costs are being estimated, with initial costs expected to be around 2-10 billion baht per km. The construction should take two to three years to be completed.
In its feasibility study, the OTP looked into underground expressway projects in Japan, Malaysia, Australia and the US.
Transport Minister Arkhom Termpittayapaisith yesterday said building underground roads is a part of the government's efforts to tackle Bangkok's traffic problems as space on and above the ground becomes more scarce.
Meanwhile, the Expressway Authority of Thailand (Exat) is gearing up to demolish a market on Kaset-Navamin Road to make way for the construction of an expressway.
Exat governor Suchart Chonsakpipat said the agency had scrapped the rental contract months ago but Talad Hua Moom market is still in business.
The area is needed for the 12km long N2 section of the planned expressway, which will connect it to the eastern ring road.
He said Exat will put up signs to instruct the vendors to leave.
The 17.5 billion-baht project has suffered long delays.
Pillars were built along Kaset-Navanim Road more than a decade ago for the 42.9km expressway, which upon completion, will link Bang Yai in west Bangkok to the eastern motorway project.
