Transport Quebec is limiting traffic on the Mercier Bridge until the fall, while work is done to repair rusted-out sections of the structure.
Authorities closed a section of the bridge Tuesday night, cutting off normally very busy lanes that link Montreal to the South Shore.
Transport inspectors discovered problems with ten steel gussets that hold together bridge beams.
The metal plates showed advanced corrosion. rust.
The repairs are unavoidable but the bridge is still safe and can be used for travel, insisted Quebec transport minister Sam Hamad.
"There is no risk in our operation. We are working in safety," he said.
As of midnight Tuesday, two lanes heading into Montreal will be open for morning rush hour.
Starting at noon on weekdays, the direction will be flipped, and all traffic on the bridge will head southbound.
On weekends, one lane will be open in each direction.
Closure constricts commuters
The unexpected closure angered South Shore communities that use the bridge daily.
Municipal leaders weren't given enough notice to prepare commuters, said Chateauguay mayor Nathalie Simon.
"It means a lot of of detours, lots of time lost on transportation instead of being with their families, with their children. It means a lot of money lost in time, in gas. It's a nightmare."
The summer commute will be a nightmare, predicted Chateauguay resident Jacques Dumouchel, who takes the bridge every day to work for a ventilation company.
"I got two jobs on the West Island this morning, and I gotta come back after it, so imagine. I don't know what time I'm gonna finish. It's incredible. I think I'm gonna hang myself somewhere. It would be better."
The AMT is adding six new daily week day departures on the Candiac commuter train line, to provide South Shore commuters another option to get into the city.