A federal judge has ordered Caltrans to correct errors in its survey of trees that could be affected by a controversial plan to widen Highway 101 along a one-mile section that winds through Humboldt County's Richardson Grove State Park.

The decision could further delay the widening, although details were not immediately available.

"There are a number of discrepancies and omissions that raise serious question about whether Caltrans truly took a hard look at the effects of the project and made an informed decision," U.S. District Judge William Alsup wrote in his ruling issued Wednesday.

They include erroneous tree measurements and locations and omission of a 48-inch diameter tree, he wrote. The survey included more than 1,500 trees, Caltrans officials said.

Alsup also declined to lift a July injunction halting the project and told Caltrans to reconsider its finding that the project would cause no environmental damage.

The ruling also contained rulings favorable to Caltrans. Alsup denied the environmentalists' request for a full environmental impact report and for monetary sanctions against Caltrans. The plaintiffs include the Environmental Protection Information Center. The action was filed in 2010.

Caltrans asserts it's allowed to conduct a lesser "environmental assessment" report for the road widening project because it will not harm any of the old-growth redwoods. The project will require removal of 54 trees, six of them redwoods between 4 inches- and 19 inches in diameter, according to the court ruling. The roots of another 68 trees would be affected, according to Alsup's ruling.

The road project is aimed at allowing large, industry-standard trucks to travel Highway 101 from Humboldt County to counties to the south. The road would remain two lanes and the speed limit would continue to be 35 mph.

Proponents of the project say northern Humboldt County is disadvantaged by the narrow road. Large trucks traveling from Oakland to Eureka currently must take a 446-mile detour that includes traveling on Interstate 5 through Oregon, then back south on Route 101, according to the ruling.

You can reach Staff Writer Glenda Anderson at 462-6473 or Glenda.anderson@pressdemocrat.com.