Steve Flint, a board member for the nonprofit, said the group’s concerns have not been quelled since members last took legal action against the state in 2003.

“One of the biggest concerns is that the eastern route that they proposed is closest to the prairie remnants,” he said.

In March, the Idaho Transportation Board unanimously approved the eastern route alternative for the 6.34-mile stretch of highway, and the formal record of decision was published in April in the Federal Register. The project will expand the highway from two to four lanes, including a 34-foot median and center turn lanes, as well as curbs, gutters and sidewalks at the northern end of the project, just south of Moscow.

The eastern alignment, one of three options considered, is nearest to Paradise Ridge. The area contains some of the last remnants of the Palouse Prairie, Flint said, including the endangered Spalding’s catchfly.

The lawsuit argues environmental damages caused by the eastern route, along with mitigation efforts, have not been adequately analyzed.

The FHA signed the formal Record of Decision on March 21 authorizing ITD to purchase land and begin construction of the project as early as the fall of 2017.

The FHA and the ITD evaluated other alternatives for highway realignment, including western, central and eastern routes, before choosing the easternmost route after years of an environmental review process, according to information on ITD’s website.

PRDC has said the proposed route would lead to loss of wetlands, remnants of Palouse Prairie, farmland and conservation reserve; and the removal of acres of pine stands and related habitat. It also would cause more noise, have a wider visual effect and it is not the safest route for travel because it is at a higher elevation than other routes and is used by large game for crossing.