The proposed by-pass for Waterford city is likely to be re-routed around the Woodstown archaeological site, the Minster for the Environment, Mr Roche indicated today.
Speaking at the site of what is regarded as a Viking settlement of international importance, Mr Roche said he was minded to direct that the site be preserved.
The National Roads Authority (NRA) has drawn up a contingency plan for changing the route after archaeologists working on the by-pass in 2003 unearthed Viking artefacts on a site which later proved to be much larger than expected.
The site at Woodstown, on the western outskirts of the city, has been described as "the most significant new find in Viking studies in perhaps a century" by Viking expert Prof Donnchadh Ó Corráin of University College Cork.
Artefacts already recovered from the area date from the ninth century and include silver ingots, Byzantine coins and Viking weaponry. A warrior burial site and a fortified ditch protecting the settlement were also discovered.
Today's announcement was expected as the NRA had advised the Minister that re-routing would be as less expensive and viable option.
Mr Roche's decision is the first time an instruction to preserve a site has been issued under powers added to the National Monuments Act last year.