England's first-of-its-kind eco road scheme has been unveiled on the A38 near Buckfastleigh.
Engineers have spent months tunnelling under the carriageway to install new drainage and environmental protections.
It's to prevent oil and residues from cars ending up in the nearby Potter’s Wood Site of Special Scientific Interest and rivers.
A spokesperson for Highways England said:"The trial project involves installing underground drainage pipes - tunnelled under the A38 at Dean Burn in Devon - and a filtration pond filled with engineered soil to trap oil and metal residue that are carried in the water running off the busy road.
"The area is particularly important because right next to the road sits Potter’s Wood, a protected Site of Special Scientific Interest (SSSI), and as well as improving water quality, earth excavated from the pond and tunnel is being used to create bug hotels, butterfly scrapes and bee banks to benefit biodiversity.
"The scheme, enabled by Highways England’s Environmental Designated Funds, has been designed by contractors Kier and delivered by South West Highways, and if successful, the system could be rolled out at other locations around the country’s major A roads and motorways."
Highways England Project Manager Michelle Reed added: "We're very excited to be trialling this new and innovative environmental system and if it’s successful we could be rolling it out across the country.
"Our Designated Funds programme was developed so that we can invest in projects beyond our traditional road build and maintenance, and this is a glowing example of how this funding can have a positive impact on people and communities.
"The filtration system provides a physical barrier to polluted water, then chemical and biological mechanisms work in combination to break down even more pollutants. It also has the advantage of taking up far less space than other treatment systems, which makes it very cost effective.
"When completed, this work should significantly improve the quality of water running into Dean Burn and help to support the local environment and its wildlife.
"This is the first time we have used this machinery in the South West and it’s been such a success we have completed the tunnelling ahead of schedule and only two overnight lane closures have been needed."
Click play to hear Nick Reed, who has been overseeing the project, speaking to reporter Andrew Kay