SALEM — Widening Interstate 93 would probably increase the population in New Hampshire by only a fraction of the thousands of people previously estimated, according to preliminary research from the state Department of Transportation.

"I-93's been in place for 40 years or more ... so it's not like when the interstate was first put in," said Larry Pesesky of the Louis Berger Group Inc., a consulting firm hired by the state.

That prediction comes as part of a court-ordered environmental review of the planned highway-widening project — a proposal to expand the highway by 2017 from four lanes to eight along a 20-mile route stretching from Salem to Manchester.

Last night, state officials presented their early work to a group of about 15 people at Salem High School. The event was part of an outreach program that is to culminate in a "supplemental environmental impact statement" this fall.

The statement is a necessary step before receiving federal approval to move forward with the highway expansion.

The new data, from the state Office of Energy and Planning, shows that population growth slowed after the initial estimates were made in 2001.

Expanding I-93 is only expected to add about 4,000 people to the state by 2020, according to the new figures. The earlier estimate predicted more than 35,000 people.

The new estimates also predict a substantial reduction in traffic on other nearby roads. Portions of Route 111, for instance, are expected to see traffic volumes cut nearly in half by 2020.

But the new estimates aren't all good news.

A highway's ability to handle traffic volumes is measured on a six-point scale, from A to F. Without the expansion, much of I-93's 20-mile stretch, from Salem to Manchester, will receive E or F grades by 2020.

But even with the expansion, the highest grade the highway will get is a C. And, by 2030, the highway is expected to have a grade of E, the second lowest, between the state line and Exit 1.

Despite that, the highway needs to be expanded, according to project manager Peter Stamnas.

"We need to move on fixing it. It needs attention and it needs it now," he said.

The possible widening has been in dispute since the Conservation Law Foundation, an environmental group, filed a lawsuit against the state in 2005.

The group won a small victory in August when a federal court judge ruled the state hadn't done a good enough job predicting the impact — in terms of vehicles and people — if the highway is widened.

The Conservation Law Foundation argued that the highway would be out of date almost as soon as it was built, and the only real solution would be to invest more in mass transit.

"The court's decision really focuses in on a critical issue and that is, what is the true benefit of this major, public investment?" said Tom Irwin, staff attorney for the environmental group, shortly after the court ruling.

The state is planning for a railroad line running from Manchester-Boston Regional Airport to Boston, but it isn't clear how soon funding for that project would be available.

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