The promise of free public transport is an enticing one: fewer cars, less congestion, less pollution.

And a greater sense of community, says Judith Dellheim from Berlin's Rosa Luxemburg Foundation.

"It could make the cities more human and more attractive," she says.

Dr Dellheim sees free public transport as a human right, not just a public good.

"This is a valid democratic issue because public transport brings people of very different social groups together, it improves the social climate," she says.

But do the promises stack up? And would free fares really persuade people to embrace public transport?

All eyes on Luxembourg

While most cities offer various concessions for public transport, no major urban centre has opted to do away with ticketing.

The exception is Luxembourg, which will abolish all fares from next month.

From April, bus and train journeys in Luxembourg will be free. ( Getty: Harald Tittel )

The European city-state is tiny, with just over 600,000 residents, but its decision has drawn huge international interest.

"It's possibly the first example of an entire region, in this case a city-state, making public transport universally free," says public transport advocate Tony Morton.

"There have been experiments in the past where various cities have introduced free public transport in their central areas. They've introduced systems where maybe the city buses are free, but the trains aren't.

"Or they've made public transport free for registered residents, but not necessarily for visitors. Luxembourg is the first example at scale."

The Estonian experience

How successful the policy change will be won't be known for at least a couple of years, but it is possible to make an assessment based on the experience of others.

In 2013, the Estonian capital Tallinn opted to abolish transport fares for all registered city inhabitants, but not for tourists and other non-residents.

Residents of Tallinn were given free travel on public transport in 2013. ( Getty: Raigo Pajula )

The move was politically popular but the results were mixed, according to Oded Cats from the Delft University of Technology.

Dr Cats, who spent several years evaluating the initiative, says there was only a moderate lift in public transport patronage, with no corresponding decrease in car use or traffic congestion.

"People that already used public transport used it more frequently, as well as people shifting from walking and cycling to using public transport for short trips, which is, of course, not a desirable effect," he says.

While the policy has been socially beneficial for the unemployed and people on low incomes, Dr Cats says the same level of assistance could have been provided through targeted concessions.

And he predicts Luxembourg's transport authorities will have a hard time persuading people to give up their private vehicles.

"About half the people working in Luxembourg commute from neighbouring countries. Many people will have to still use legs of a trip which extend beyond Luxembourg, meaning that the trip is not completely free," he says.

Existing workplace incentives, like employer-guaranteed parking spaces, will also make eliminating private vehicle use difficult, he says.

Service trumps price for transport users

Some argue lowering car ownership is more important than making public transport free. ( ABC News: Luke Rosen )

Mr Morton, who is the president of Melbourne's Public Transport Users Association, is also sceptical about the Luxembourg experiment, and about the broader notion that ticket pricing is the main barrier to increased public transport usage.

"We've tended to argue that public transport needs to be cheap, but it doesn't necessarily have to be free," he says.

He says scrapping fares won't persuade people to embrace a service which they experience as deficient or poorly run.

"We haven't really made public transport a viable, attractive mode of travel for people living in the suburbs of our capital cities," he says.

"The question of how much it costs to get on the bus or on the train is not even relevant because that bus or train service doesn't exist where they are."

Transport economist Ansgar Wohlschlegel warns the introduction of free public transport could have perverse results if it isn't paired with complementary measures aimed at driving down car ownership.

"Once people start moving from car driving to using public transport, then the roads get less congested, therefore car driving becomes more attractive again, and therefore new people may start using the car to drive into the city because now the roads are clearer," he says.

And that, says Dr Wohlschlegel, could ultimately result in the worst of all outcomes: increased public transport demand, coupled with an eventual increase in car traffic.

Sorry, this video has expired As Australia's cities keep growing, so too will demand for public transport

Dr Cats agrees.

What's most important, he says, is making car use more expensive during those parts of the day associated with congestion.

"That has to do with parking fees; in city centres it has to do with congestion charging, with fuel taxes — unpopular measures, of course, but those are the most effective measures for reducing congestion," he explains.

"Secondly, improving the quality of public transport, specifically at those times of the day in those areas, and building very strong, high-capacity urban rail systems."

Adjusting for the peaks and spreading demand

For international transport consultant Jarrett Walker, demand-responsive pricing is fundamental to the efficient movement of commuters in already congested cities.

"Public transport agencies need to encourage people to travel outside rush hour if they can, because service at rush hour is very expensive, and outside of rush hour you have surplus capacity," he says.

Fares, he says, are a simple and effective means of limiting rush-hour movements.

But he argues for greater flexibility in non-peak times.

Mr Walker says making travel free during those periods could help spread demand more evenly and have a positive social impact, particularly for those on low incomes.

"They are more likely to be travelling all over the clock, and they are least likely to be travelling into the city in the morning and out of the city in the afternoon," he says.

"It's the difference between having a job in a bank and having a job at Hungry Jack's or at McDonald's, or something like that, where you are coming and going all over the clock."

Flexible pricing can be used to limit rush-hour movements. ( ABC News: Danielle Bonica )

When a technology 'cure' becomes part of the problem

Mr Walker is also sceptical about the role ride-hailing services like Uber and Lyft can play in dealing with urban congestion.

App-based car-hire companies often market themselves as an answer to traffic congestion and as a complement to public transport.

But Mr Walker says the full picture is far less optimistic.

"If a new ride-sharing solution gets two or three people in a little vehicle, that's better than those three people driving cars. But it's worse than those three people riding the bus or train."

And new research from the University of Kentucky suggests a correlation between the rise of ride-hailing services and a decline in public transport patronage in the United States.

Transport engineer Gregory Erhardt surveyed publicly available transport data in 22 metropolitan areas.

"There have been theoretical arguments saying that Uber and Lyft bring people to and from the rail stations," he says.

"That perhaps they are concentrated at night, bringing people home from bars when transit doesn't operate, and so forth.

"What we found is that that's not the case. In fact, they are operating often in the peak periods, they are operating in places where they are concentrated in the city centres, in the exact same places where public transit is viable."

He estimates the effect on public transit has been significant. Over a six-year period, companies like Lyft and Uber, he says, can reduce heavy rail ridership in a city by as much as 7.5 per cent, and bus ridership by almost 10 per cent.

And that means more, rather than less traffic.

"But there is a clear benefit to the person in the car: they have this door-to-door experience that you don't get in public transit," Mr Erhardt adds.

Experts say we need to have a realistic conversation about the cost of public transport. ( ABC News: Luke Rosen )

Looking forward, Mr Morton argues we need a more realistic conversation about the cost of investing in better public transport, balanced against the enormous amounts of public money spent enlarging and extending road networks.

"The stated motivation for not wanting to expand public transport and to boost its use is that public transport is a drain on public funds, whereas it is thought that roads somehow pay for themselves," he says.

"Now, roads do not pay for themselves. There's actually quite a substantial public subsidy for the road transport system as well."

For Dr Dellheim it all comes back to one thing.

"When the whole of society is fixed on cars, then of course the whole life of the society, the whole economy of the society, is oriented on the car industry and car use," she says.

"So, it means that it's necessary to rebuild the whole life of the society, to show the people that there are different possibilities, and then you see that there is a real desire to change the mode of life of the society."

But whether free public transport is one way of doing that remains an arguable point.