Planners could not have picked a worse route for a dual carriageway near a protected wetland, a court has been told.
Environmentalist Chris Murphy is opposing plans to dual the A6 at Toome.
The proposed road is close to Lough Beg, a protected habitat for migratory birds including overwintering swans.
The plan has also caused controversy as it runs through a landscape celebrated in the poetry of Nobel Laureate Seamus Heaney who was raised nearby.
Mr Murphy, who is representing himself in court, described himself as "a bird watcher not a lawyer"
He is contesting the decision to proceed with the work on the basis that the proper assessment of its impact has not been done in line with EU legislation.
The case is being contested by the Department for Infrastructure which wants to build the £160m stretch of road.
Mr Murphy told the court the department had picked a route that was "most engineeringly difficult and most culturally and environmentally damaging".
He added: "They couldn't have picked a worse route."
Mr Murphy is making the case that environmental information underpinning the decision to proceed was way out of date.
He said the surveys were based on information more than a decade old.
The department ought to have conducted a fresh assessment of the potential impact of the project before going ahead, he added.
For such environmental information to have value it had to be the "best available science" and "up to date".
He said the department for infrastructure had tried to "circumvent" the strict procedures in such cases.
Department officials had been told, he said, but had ploughed on "recklessly".
The A6 at Toomebridge is a major bottleneck on the main Belfast-to-Derry road.
Supporters of the development say the dual carriageway is needed for economic development and safety reasons.