PORTLAND – Maine Turnpike Authority officials are working to keep the highway off a federal list of historic roads, bridges and tunnels.

The highway has been included on a preliminary list of historically significant parts of the federal interstate system that would give it national historic status. The list was developed by the Federal Highway Administration.

Although they haven’t fully assessed what impact such a designation would have, turnpike officials are concerned that being listed on a federal register could hamper future construction projects.

“It would probably restrict improvements we could make to the highway and would probably make them significantly more expensive,” said Maine Turnpike Authority spokesman Dan Paradee.

The FHA’s 13-page list is being circulated in connection with the 50th anniversary of the U.S. interstate highway system.

Nationally, most of the interstate highway system was exempted from consideration for historic preservation, but there were exceptions.

The draft list says the Maine Turnpike is the nation’s second-oldest turnpike and that it has been designated as a landmark by the American Society of Civil Engineers.

The highway, the first section of which opened in 1947, runs from Kittery to Augusta.

Famous landmarks on the list include the Holland Tunnel between Manhattan and New Jersey, Alligator Alley in the Florida Everglades and the Bay Bridge in Northern California. Turnpikes in Pennsylvania and West Virginia also are listed.

Turnpike officials have shared their concerns with members of Maine’s congressional delegation, the state Department of Transportation, municipal planning organizations and members of the Legislature’s transportation committee, said Conrad Welzel, who heads government relations for the turnpike authority.

“We’re working very hard at clearing it up,” said Welzel. “And I feel that people recognize our concerns.”