MONTREAL - Nearly one-third of Montreal's deteriorating road structures need repair work and a dozen of them are considered to be in critical condition, according to a study released Wednesday.

The report examined 555 pieces of infrastructure under local jurisdiction. It concluded 12 were in the most critical shape, 44 were deficient, 38 were mediocre, and another 81 were just generally deteriorating.

That means more than 30 per cent of the 555 structures such as bridges, tunnels and overpasses need work.

The city's chronic infrastructure problems were brought into focus this summer when a giant concrete slab collapsed onto a downtown expressway, while bridge closures caused monster traffic jams.

Mayor Gerald Tremblay sought to reassure residents who are becoming increasingly rattled by the prospect of vital infrastructure collapsing around them.

Some of the 12 most-damaged structures are either fully or partly open, but Tremblay said there's no reason to fear them.

"If an engineer were to tell us in a very clear manner ... that infrastructure is not safe, we would close that infrastructure," Tremblay told reporters. The mayor also defended his administration's general handling of the issue.

"It's true that we inherited a city with dilapidated infrastructure... There's a tremendous amount of catch-up to do because our structures were seriously underfunded for decades."

Theories abound about why the city's roads are in such poor shape. Two factors commonly cited are improper maintenance and the use of poor materials in the original construction.

Some public-policy analysts say, however, that Montreal's woes highlight an increasingly common problem across the country: improper funding for city infrastructure.

Montreal says it needs to nearly double its spending on infrastructure refurbishment over the next three years -- to just over $50 million annually for a total of $157 million -- from the current level of $30 million a year.

But the mayor says that will only cover the most critical problems.

He said Montreal's crumbling roads need an extra $200 million and, with the municipal budget badly squeezed, he made it clear he expects other levels of government to help.

Tremblay said he hopes to hear some commitments from the other levels of government over the coming weeks or months.

"Events like the ones we've recently experienced create what I'd like to call, 'useful tension.' Now, there's useful tension all over the place," Tremblay said.

"There's useful tension at the federal level, there's useful tension at the provincial level and there's useful tension in Montreal and in other cities. When there's that useful tension and people start having the perception that infrastructure isn't safe, that hurts economic development, it hurts social development and it hurts the sustainable development of Quebec's metropolis."