Construction crews working overnight on the Champlain bridge were unable to finish their work before rush hour began Wednesday morning.
They ran out of asphalt and concrete to patch holes that they had made in the bridge decking.
As a result, their work created several large bumps between the plates of the bridge deck.
The bumps were so high, and so steep, that four cars and a motorcycle travelling at highway speeds were disabled near dawn.
Nobody was injured in any of the accidents.
Instead of repairing the bridge deck immediately and creating bumps that were less steep, orange cones were used to block off lanes and slow down traffic.
The resulting bottlenecks caused lengthy delays for south shore commuters trying to get to Montreal.
By 7 a.m., the line of vehicles on highway 10 west stretched back more than six kilometres, to highway 30, and it was taking drivers one hour to move that short distance.
Many drivers decided to use other bridges, which caused backlogs on those spans as well.
The bridge corporation said it was an unusual mistake.
"We have a lot of that kind of work last year, last week and normally everything goes well," said Jean-Vincent Lacroix.
"We want to make sure than it won't happen again and I can tell you it won't happen again."
More construction is, as normal, planned all summer-long on various bridges into Montreal.
