LEWISTON — A long-studied downtown Lewiston-Auburn turnpike interchange wouldn’t fix Twin Cities’ traffic patterns enough to justify building it, traffic engineers told city councilors Tuesday.

“There just is not enough benefit from a new Exit 79,” said Duane Scott, director of multi-modal planning at the Maine Department of Transportation. “It just doesn’t help enough to justify the expenditure of public dollars.”

Instead, representatives from the MDOT, the Maine Turnpike Authority and traffic engineering firm Fay, Spofford and Thorndike suggested the area would be better served by making Lewiston’s turnpike Exit 80 more efficient.

State transportation officials said they have $20 million set aside to help make Exit 80 work more efficiently. Work on building new on-ramps could begin in 2011 and complete improvements to the turnpike exit and the nearby Plourde Parkway-Lisbon Street interchange could be finished by 2015.

The Androscoggin Transportation Resource Center has been studying the need for a new turnpike interchange since 2002, hoping it would encourage economic development in both cities’ downtowns. The resource center had settled on three plans — building new interchanges on River Road in Lewiston, on Route 136 north of the turnpike in Auburn or on both sides of the river.

But the distances between the proposed interchanges show that they would not help speed traffic downtown, according to traffic engineer Edward Hollingshead. Currently, northbound traffic headed downtown, exiting in Auburn must drive 4.7 miles along Washington Street to reach downtown. Northbound traffic exiting in Lewiston must drive an additional 4.5 miles between the two cities, then drive an additional 3.4 miles to reach the downtown.

A new interchange in the middle wouldn’t shave off any distance, since drivers would still need to travel nearly 3.5 miles between the Auburn and downtown interchanges, then another 3 miles along the river to reach the downtown.

Since it doesn’t save drivers time, it wouldn’t encourage development downtown.

“What we found out is those three options don’t meet your needs and can’t help attract new development because they don’t reduce travel times,” Hollingshead said.

Traffic congestion would be better served by cleaning up Lewiston’s Exit 80, according to traffic engineer William Moore. He suggested changing the interchange between the turnpike and Plourde Parkway into a diamond design, similar to the Sabattus turnpike interchange.

“The loop ramps you have there now require cars to slow down too much and there’s a potential there for traffic backups,” he said. Straight on and off ramps would be more efficient.

He also recommended consolidating the Plourde Parkway traffic heading onto Lisbon Street, north and south,  into a single intersection with a traffic light.

Moore estimated those changes, to both the turnpike and Lisbon-Plourde interchanges, would cost $20-$25 million.

Scott, of the MDOT, said his agency has $5 million earmarked for the Lewiston interchange. The Maine Turnpike Authority has another $15 million earmarked for Exit 80 improvements.

Engineers are scheduled to continue discussing the study, including presenting it to the Auburn City Council on Monday. A public review is also scheduled and the final report is due in September.

staylor@sunjournal.com