A Montreal think tank is suggesting public transportation in the city should be free to ease traffic headaches and cut greenhouse gas emissions.

The group points out free transit would cost a lot less than the subsidies already given to car drivers.

A new report from Institut de recherche et d'informations socio-économiques (IRIS) was released Thursday - and timed to coincide with the general elections to generate discussion.

IRIS said free bus and metro rides for everyone would cost the city $620 million -- "that's the part the users of the STM are paying right now" -- but would be offset by other benefits.

"The experiences of free public transit elsewhere in Quebec and internationally show that they have undeniable impacts on traffic flows, improving the quality of life and the purchasing power of public transit," said IRIS researcher Bertrand Schepper.

The price could be offset, IRIS said, if Montreal were to take advantage of some government programs for reducing greenhouse gas emissions.

It would also improve congestion, health effects, stress and quality of life if more drivers switched from driving to public transit.

He said the province should also increase funding for public transit, and Hydro-Quebec should give the metro system a better rate for electricity.

"The city of Montreal has launched a challenge to reduce its greenhouse gas emissions by 80 per cent by 2050, and if the future elected members really wish to achieve these results, access to public transit must become a priority," said Schepper.

Schepper said it is also cost effective -- a person driving a car is being subsidized at a rate more than six times higher than someone using public transportation.

IRIS says if Montreal isn't prepared to make public transit free across the board, there are ways to do it bit by bit, starting with certain populations.

"If students and older people would have free access, it would cost around $107 million. If, for example, only people getting on the Montreal area, on the island, would use public transportation, that would cost $507 million," said Schepper.

Mayoral candidate Valerie Plante said her party, Projet Montreal, has already said it is in favour of free transit for students and seniors.

"We will support that, free public transit for youth and for elders and this is what we're going for," said Plante.

Mayor Denis Coderre said he supports the idea in principle, but said it would be difficult to implement.

"We will take a look at it but I would suggest that in the near future to say that's it's going to be free... Where the revenue will come from? I mean $620 million, we need to be realistic," said Coderre.

The small cities of Candiac and Ste. Julie already offer free bus service as a way to attract residents, while Longueuil offers seniors free bus rides outside of rush hour.