Conservation groups have sued to stop the construction of the last of the 540 loop in Wake County.
The N.C. Department of Transportation plans to start building the rest of the 540 loop from Holly Springs to Knightdale next year.
But the lawsuit claims the U.S. Fish and Wildlife Service did not properly analyze the project's environmental impacts. In particular, there are two species of endangered freshwater mussels that could be at risk.
Wildlife officials have said there are enough construction safeguards in place to protect them. The agency issued an opinion in April that said the toll road would have no significant impacts on the dwarf wedge mussel or the yellow lance mussel.
Attorney Kym Hunter of the Southern Environmental Law Center said the opinion is flawed.
"Often those procedures are not properly followed, but more importantly, what we're really concerned about here is once the highway is in place, there's going to be a lot of additional growth and construction, and we're going to see a lot of nutrient runoff from that growth and increased impervious surfaces," said Hunter.
Neither the Fish and Wildlife Service nor the Justice Department would comment on the lawsuit.