Section of Commonwealth Avenue will close this summer for bridge work

If everything goes according to plan, rebuilding the Commonwealth Avenue bridge will take 18½ days — plus another month or so of preparation and cleanup on the Mass. Pike below.

Repairing everyone’s nerves may take considerably longer.

After years of planning and delays, a major construction project to replace the aging, structurally deficient bridge could snarl traffic all over the city when it begins in late July, state officials warned on Monday.

It will affect nearly every mode of transportation, altering bus and train routes and eliminating most traffic on a 1.5 mile stretch of Commonwealth Avenue and the Boston University Bridge, and choking the turnpike below.

For some, an 11-day stretch when the number of lanes on the Massachusetts Turnpike will be halved will be the worst of it.

“Two weeks?” one turnpike traveler said on Monday. “It’ll feel like two months.”

And if the city survives it, we get to do it all again in 2018, when the second half of the nearly $82 million project is planned. The project is already starting a year late after a design error prevented work from beginning last summer.

“We’re going to have some problems” if motorists don’t make plans to avoid affected areas, said Thomas Tinlin, the state’s highway administrator. That means somehow skirting several major routes into and out of Boston and Cambridge.

By compressing the construction timeline — using fast-drying concrete, working around the clock, aggressively closing roads — the Transportation Department could fit four or five years of unpleasantness into a few weeks, Tinlin said.

But those few weeks could be extremely unpleasant.

Interstate 90 will lose two lanes of traffic in both directions for nearly two weeks; a stretch of the Green Line will be replaced by shuttle buses; and two weekends of commuter rail service to Worcester will be affected, along with Amtrak’s Lake Shore Limited service, with buses replacing trains for portions of each journey (July 29-30 and Aug. 5-6).

After a June 2-4 dry run to test traffic flow, during which the Mass. Pike will be narrowed to two lanes in each direction, the setup for the bridge project begins on July 7, when travel is reduced to three lanes in each direction between the Beacon Street overpass and the Allston interchange.

That drops to two lanes each way from July 28 to Aug. 7, when all drivers will be routed onto one side of the turnpike while work is completed on the other. Midway through that 11-day period, all traffic will switch to the other side, so the work can shift. It will be back to three lanes each way until Aug. 28, when the work is scheduled to end for the year.

Meanwhile, Commonwealth Avenue will close to through traffic in both directions from Kenmore Square to Packard’s Corner, July 27 to Aug. 14. Traffic to businesses will be permitted, and the area will be open to pedestrians and bicyclists.

And the Boston University Bridge over the Charles River will be closed to all nonemergency, non-transit motor vehicles during the same period but be open to pedestrians and bicyclists.

“This time frame was carefully selected to minimize traffic impacts,” Tinlin said. Indeed, weekday ridership on the Green Line’s B branch, which runs down the center of Commonwealth, drops to 10,900 daily in the summer, according to MBTA statistics — less than half of the annual daily average.

The dates also avoid college move-in and the Fourth of July, though the Red Sox play home games during much of the construction. Buses will serve the Boston Landing and Yawkey commuter rail stations during Sox games.

On Monday, commuters expressed some understandable skepticism that modern construction magic might somehow turn another years-long concrete quagmire into a brief inconvenience.

But to be fair, driving down Commonwealth Avenue is no picnic even in the best of times. On Monday afternoon, a skateboarder beat a reporter in an SUV from Packard’s Corner to the interstate overpass by several hundred yards. Notably, he didn’t even know it was a race.

And that was before a fire in a Boston University building shut down westbound traffic on Monday and filled the road with fire engines — one of which was struck by a bus.

But closing down the road is, for some, a bridge too far.

“We’re already crying,” said Emile Heraiki, who manages the busy Mobil station on the corner of Commonwealth and Amory. Much of Commonwealth will remain open to local and business traffic, but it’s hard to imagine many motorists braving the construction zone to get gas.

“I don’t think it’s going to be that busy,” Heraiki said. “We’re going to be in trouble.”

Still, ripping off the Band-Aid, as Tinlin referred to the strategy, could be preferable to the seemingly endless construction cycles with which Boston is familiar.

“It’s got to get done. Let’s do it in one fell swoop,” said Gustavo Quiroga, director of place-making at the retail consulting firm Graffito SP. He said businesses may be better equipped to handle a few weeks of major disruptions than months or years of protracted road work.

“There’s a natural tendency to proclaim the sky is falling,” he said. “And then people find alternate routes.” Perhaps, he said, they’ll find new ways to get where they’re going — a new shortcut by car or switching to public transit or a bicycle — and stick with them even after the work is complete.

“Would I prefer to have the construction done in January? Sure I would. I run a seasonal business,” said Mark Vautour, manager of Landry’s Bicycles on Commonwealth Avenue.

And though most of the store’s employees commute to work by bike, many customers — particularly those buying expensive bicycles — come by car.

But Vautour said he understood the state’s strategy in scheduling and planning the project, which includes improved bicycle infrastructure in an area that has seen several cyclists hit by cars and trucks.

“I believe in the long term it’s a good thing for the city,” Vautour said.

And though he often drives to work so he can get to and from Landry’s other stores in the area, he tries to remember that the cause of gridlock is often in the mirror.

“You’re not stuck in traffic,” he said. “You are traffic.”

Nestor Ramos can be reached at nestor.ramos@globe.com. Follow him on Twitter @NestorARamos.