The underpass was under 20ft of water at one stage
A clear-up operation is continuing after flooding across Northern Ireland.
The emergency services are continuing to work to clear the Broadway underpass in Belfast, which at one point was submerged under 20ft of flood water.
Traffic managed to flow without major delay along the Westlink, with M1 users heading along the Broadway roundabout.
The underpass is expected to remain closed all day Monday. An investigation has been ordered by Roads Minister Conor Murphy into how it flooded.
Mr Murphy visited it over the weekend.
It is thought a pumping station beside the road was overwhelmed when a nearby river overflowed.
"I have asked Roads Service to carry out a full investigation into the underpass' flooding," he said.
"The contractors are still working here and we will want to be talking to them to ascertain how it became flooded and why it didn't clear itself quickly as well as to make sure that this sort of thing doesn't happen in future."
A section of the central barrier of the adjoining Westlink had to be cut with angle-grinders to allow motorists to flee flood waters.
It is thought up to half a dozen cars may be submerged in the underpass, having been abandoned by their owners as the water rose.
Mr Murphy said the relevant agencies met on Friday after receiving weather warnings.
"As a result Northern Ireland Water was able to deploy resources and contractors throughout the event and around the clock," he said,
"Nevertheless, such was the extremity of the rainfall, that hundreds of homes were affected."
It is thought that the road will reopen on Tuesday.
The clean-up operation will continue in hundreds of flooded homes.
A house became an island near Lurgan during the floods
Nearly a month's rain fell in just a few hours on Saturday afternoon causing rivers to burst their banks in counties Antrim, Down and Londonderry.
Brian Kennedy's home was flooded when the Sixmilewater overflowed.
"The fridge will have to be replaced, the washing machine, tumble drier - all gone," he said.
Environment Minister Sammy Wilson announced a compensation package for anyone whose home was affected.
"First thing in the morning that compensation package will be available to councils," he said.
"Councils will know what they have to do, individuals will know how they can apply through councils and get the environmental health officer out to verify their property was affected."