Calling it "kind of the worst kept secret," the Massachusetts Department of Transportation's highway administrator told his board of directors on Tuesday that restoration of the Longfellow Bridge has fallen behind schedule and is now expected to last until at least November 2018.

The setback means people who rely on the historic bridge, known for its "salt and pepper shaker" towers, to commute between Boston and Cambridge may be in for about three more years of delays, detours and headaches.

"You don't have to be a construction expert to realize that this thing was going a lot slower than people had anticipated," Thomas Tinlin, MassDOT's acting highway administrator, said.

Tinlin said the project's contract mandated the use of "complex techniques that have fallen out of use in today's construction industry" like riveting rather than bolting and welding. The project was also set back by significant deterioration of some of the steel in the bridge that was not obvious to engineers until the bridge started to be disassembled.

"The rehabilitation of this historic bridge has not been an easy or straightforward process," he said. "The Longfellow has been complicated by a number of factors that boil down to the fact that restoring this historically significant structure back to its former glory has required the use of construction techniques that are no longer the standard in 2015 and have taken more time to do and to do right."

The project management team for the bridge reconstruction will host public meetings in Boston and Cambridge in September to lay out the new construction timeline to residents and commuters.

The Longfellow Bridge, at its regular capacity, carries 28,000 vehicles and 90,000 riders on the MBTA's Red Line between Boston and Cambridge each day, Tinlin said.

When it started, the Longfellow Bridge restoration had a projected budget of $304 million and was expected to be completed in 2016. Tinlin said Tuesday the new timeline calls for the bridge to be fully open to traffic by November or December 2018.

"We've told the contractor to really think outside the box and we've challenged them on how they're going to try to make some lost time," Tinlin said. "We've also asked them to bring on additional crews and additional equipment in order to accelerate the bridge reconstruction."