A coalition of environmental groups has dropped its lawsuit challenging a controversial Highway 101 widening project through a grove of Humboldt County old-growth redwood trees after Caltrans decided to seek further environmental review of its plans.

Opponents of the 1.1-mile project through Richardson Grove State Park say the work would damage the roots of the protected redwood trees. Caltrans says the $5.5 million project is needed to alter the narrow section of highway — one of the last remaining bottlenecks on the North Coast’s main thoroughfare — to meet federal standards for large trucks.

Environmental groups, including the Center for Biological Diversity and the Environmental Protection Information Center, filed their federal lawsuit in July. Caltrans last month decided to withdraw its environmental findings and begin anew with a second review. The process could take two years to complete, Caltrans spokesman Phil Frisbie said.

He said Caltrans made no deal with environmentalists to get them to drop the lawsuit.

“We came to this decision on our own, and then went to the court” to seek dismissal of the suit, he said.

Still, environmental groups said further study of the project’s impacts would benefit the 1,000-year-old trees. They announced their dismissal of the lawsuit on Friday last week.

“This is an important victory stopping a nonsensical project that would have done terrible damage to an ancient grove of giant redwoods in our state park,” said Jeff Miller, spokesman for the Center for Biological Diversity. “We’ll be ready to go back to court if Caltrans decides to pursue the project, and it’ll have to completely start over on environmental review and the approval process.”

Frisbie said Caltrans has not abandoned the project. He said Caltrans decided that it was in the “best interests of the state” to withdraw environmental documents for the project and “start fresh from square one with a new environmental process.”

Caltrans initially determined that there would be no negative impacts from the project, which will makes some “minor modifying changes” to curves in Highway 101 at Richardson Grove so that two standard trucks can pass safely, Frisbie said.

Environmentalists say that Caltrans should consider alternatives that are less costly and damaging to the redwoods’ root system. Natalynne DeLapp, executive director of the Environmental Protection Information Center, said traffic signals or limiting the hours trucks can use the stretch of road would improve safety.

“We believe the project is unnecessary,” she said. “Anything that cuts the roots is unacceptable.”