After some major road surface work in the past few years, Interstate 35 as it runs through Duluth is in “good shape,” said engineer Todd Campbell with the Minnesota Department of Transportation.

“Our focus is on preservation,” he said.

The “mega project” that began in 2011 from the bottom of Thompson Hill to the Can of Worms interchanges was the first major reconstruction of the road surface there since the freeway opened in 1971. Campbell said 40 years is about the expected life span, depending on techniques used on base layers. He said the road surface from Mesaba to 26th Avenue East is adequate since some grinding work, and there won’t be any major surface replacement for 20 years.

Traffic patterns are stable, and with no major growth expected in Duluth, the interstate is handling traffic capacities.

The engineering of the freeway could change, especially when it comes to the crossover exits and the flyways and bridges at the Can of Worms. It’s a design that is obsolete, Campbell said. There are 37 bridges from 27th Avenue West to Mesaba. Bridges require a lot of upkeep, Campbell said.

There are studies underway on how to more efficiently handle the traffic flows where U.S. Highway 53, 21st Avenue West and Blatnik Bridge traffic meet with the interstate.

“There are more efficient ways,” Campbell said, including eliminating the weave across traffic lanes that interstate drivers going south need to do to get on the bridge to Superior. Some of the interchanges also could be lowered to be “at grade,” Campbell said.

But any reconfiguration would be a “budget buster,” he said, so MnDOT has to choose carefully what it will take on.

“There’s still a fair amount of useful life left” on the bridges, Campbell said. “We hate to throw away any useful life.”

The pilings that hold the roadway in the area known for the Graffiti Graveyard near the Depot also are being studied. There was a lot of unstable soil when those pilings were placed, Campbell said, including sawdust, sand and silt. Today there are better technologies to build on bad soils, he said.

But any major structural work is years off.

“We have to concentrate on the road surface,” Campbell said.