It's a recurring idea — enticing to some, loathsome to others — that often ends up on the cutting room floor.

Montreal Mayor Denis Coderre floated the possibility this week of a toll system that would charge drivers heading into the city.

The extra fee would help curb congestion, he said, while the revenues could be used to help fund public transit projects.

Coderre cited the congestion charge introduced in London in 2003 as an "option that could be very inspiring."

The idea was one of several put forward by Coderre on Thursday, when the mayor announced the creation of a two-year, $3.4-million research project devoted to electronic and intelligent transportation.

The ultimate goal, he said, is to reduce congestion, encourage public transportation and foster the growth of the city centre.

A recent study found that a Montreal driver spent, on average, 52 hours in traffic in 2016. The transportation analytics firm Inrix ranked Montreal the worst place for traffic in Canada, and 23rd worst overall.

Politically taxing

​Tolls have been cited as an effective way to reduce traffic and pollution, but they can also be politically fraught.

Earlier this year, Toronto Mayor John Tory's plan to put tolls on two major highways — the Gardiner Expressway and the Don Valley Parkway — was nixed by Ontario Premier Kathleen Wynne.

Wynne said she won't approve the plan until suburban drivers have better transit alternatives to get to and from the downtown area.

Tolls and fees are more common in Europe.

Traffic signs are seen showing the entrance into London's congestion charge zone for central London. The system has resulted in an overall reduction in traffic. (Alastair Grant/Associated Press) In 2013, 10 years after the implementation of the London congestion charge, a study found that there was a 10 per cent reduction in traffic.

Moreover, the charge generated roughly $2 billion in net revenue during that time, which was poured into public transport, road and bridge improvement, as well as walking and cycling projects.

The system — in effect during peak hours between Monday and Friday — does not rely on tolls. Rather, it depends on licence recognition.

A similar model was implemented on a trial basis in Stockholm in 2006, where it led to a significant reduction in congestion. It was made permanent the next year and now brings the city roughly $107 million per year in additional revenue.

What's the big idea?

Ex-mayor Gerald Tremblay also wanted to install tolls to help fund public transit. In 2009, he said tolls on roads into Montreal from Laval and the South Shore could bring in as much as $400 million.

There was talk of putting a toll on Montreal's new Champlain Bridge. But with his eye on several South Shore ridings, Prime Minister Justin Trudeau ruled out that possibility in the last election campaign.

Coderre was also against a toll on the new bridge.

Denis Coderre says improved public transit, like the planned light-rail project, is key to growing the city. (Caisse de dépôt et placement du Québec) On Thursday, though, he said that was because the tolls were going to be used to fund the bridge, and not public transit.

Coderre didn't go into details about how a toll system or congestion fee would work.

The aim of any policy, he said, wouldn't be to introduce another tax, but rather to "encourage public transit use, especially in the downtown area."

Mark Watts, executive director of the C40 Cities Climate Leadership Group, which is involved in the Montreal research project, said the future of cities doesn't rest with cars.

"I'm afraid the era of the private automobile with the combustion engine is pretty much over and we're going to have much healthier cities, much quieter cities and ones where there's going to be a lot more as a result." he said.

By Coderre's reasoning, a toll system would help finance the transition to this car-less future.