Public transportation ridership may have increased in 2012, but major transit agencies across the nation have picked an odd way to celebrate. On July 1, the Southeastern Pennsylvania Transportation Authority (SEPTA), which serves the Philadelphia region, will increase fares. Meanwhile, New York City’s Metropolitan Transit Authority (MTA) will, for the fourth time in five years, up its prices this weekend.

Seven time zones to the east, the Estonian capital of Tallinn has taken the opposite approach: In January, it entirely scrapped fares for city residents, although they must initially purchase a smart card. Those who live outside the city still have to pay fares. According to Reuters, the city decided a carrot in the form of free rides is the best way to deal with traffic congestion by luring drivers onto buses and trams. (A bit of stick has been applied, too, as cars are now barred from some roads and parking fees have increased.) The city government purchased 70 new buses and 15 new trams to meet the anticipated surge in demand. Three-quarters of Tallinn residents support the plan.

Free fares have only been in place for a little less than two months, but so far the policy seems to have had its intended effect. Tallinn’s transportation department reports that traffic fell by 10 percent, meaning about 7,600 fewer cars in the city per day.

Officials also claim the nearby town of Keila (population 10,000), which made public transit free starting February 1, is emulating their model. Keila’s decision makes sense for an inarguable reason: The fare system for its two bus lines costs more to operate than any revenue it brought in. As Jarrett Walker wrote in his 2011 book Human Transit, many cities that offer free public transportation do so for similar reasons: They are so small that the cost of maintaining fare infrastructure isn’t worth it (or they are university towns, where a school subsidizes the cost of most rides).

As Walker notes in a blog post on Tallinn, the city is uniting with a few other Western European cities that offer free transit, all with populations well below 100,000, to form a Free Public Transport European Network and spread the idea across the continent. Tallinn is the largest city yet to provide free public transportation, but its population is still substantially smaller than that of Washington, D.C. (by about 200,000 people).

Would such a scheme work in a mid-to-large American city? According to Jennifer Perone’s 2002 study, sponsored by the U.S. Department of Transportation and Florida Department of Transportation, it isn’t likely. While she wrote that free transit is manageable and worthwhile in rural regions and small towns, she cautioned against such a policy for larger systems:

All well-informed transit professionals that were contacted for their opinions spoke strongly against the concept of free fares for large systems, suggesting some minimal fare needs to be in place to discourage vagrancy, rowdiness and a degradation of service.

These experts drew on the experiences of three mid-sized American cities that briefly experimented with fare free transit. Between 1978 and 1979, both Trenton, N.J., and Denver, Colo. offered free transit during non-peak hours. In both cases, ridership increased (by 16 percent and 36 percent, respectively). The authors claim however, that these were generally not people lured from cars, but were what they call “problem riders.” In Trenton, 92 percent of transit drivers reported that their jobs were less enjoyable after the free fare program was instituted. These issues, along with revenue problems, caused both cities to discontinue their experiments after a year.

Austin, Texas is the only sizable American city to try totally free transit, from October 1989 to December 1990. Ridership increased dramatically but it was “regarded as disastrous” due to a spike in “problem riders” and the increased expense of security measures and repair of damaged vehicles. The policy also proved unpopular with transit drivers, 75 percent of whom signed a petition to end fare free transit because of their negative experience with “Joy-riding youth and inebriated adults, as well as vagrants.” Physical assaults on Austin’s public transit grew from 44 in the three months before the institution of free fares to 120 in the three months afterwards. “In none of the [three] experiments did the increase in transit ridership include automobile commuters enticed by the fare-free service,” Perone writes.

Many of the experts Perone interviewed suggest that even very low fares could dramatically cut down on issues with “problem riders.” The Miami Beach Transportation Management Association offered free electric shuttle bus service and experienced the same kind of issues and complaints listed above. Then it instituted a 25-cent fare. Judy Evans, the agency’s executive director, claims a 90 percent reduction in vandalism as a result.

Considering how hard it is to raise money to do, well, pretty much anything in the U.S., finding an alternative source of public revenue is the other great challenge to fare free transit. New York’s Mayor Michael Bloomberg had a dreamy suggestion along those lines last year, telling reporters:

If you were gonna design, keep in mind, the perfect public transportation system, you would have it be free and you would charge people to use cars, because you want the incentive to get them to do that.

There actually is a detailed plan for just such a fare free system in New York. Promoted by the late transit advocate Ted Kheel, it proposes a 24-hour, $16 congestion charge for cars entering Manhattan south of 60th Street. The proceeds would then be used to pay for free public transportation and further discourage personal automobile use in the city. Proponents claim that such a plan would increase support for congesting pricing, as it would allow citizens to clearly see where the money paid by drivers is going.

Of course, the fate of Bloomberg’s much less ambitious congestion pricing scheme (it died in Albany) suggests that Kheel’s plan is unlikely to be instituted, even in the U.S. city most dependent on public transportation.

While Tallinn’s experiment is probably too new to provide us with much guidance, Henry Grabar writes in The Atlatntic Cities that a few French municipalities paid for free fares by raising taxes on larger businesses (the studies he parses are written in French, so I’m relying on his analysis here). In Châteauroux, a city of 49,000, ridership grew by 81 percent between 2001 (when fare free was instituted) and 2002, and has increased further since. The city of Aubagne, which is only a little bigger than Châteauroux, started providing free transit to its whole metro region in 2009 (around 100,000 people). Since then, ridership has grown by 170 percent and congestion has dropped by 10 percent. One French magazine reported that a near-perfect 99 percent of residents said they were happy with the policy.

Those last numbers seem to show that the “problem rider” issue hasn’t troubled the residents of Aubagne. In Tallinn there is a slight barrier to entry, in that riders have to buy a card and prove residency before getting free rides. This may suffice to cut down on issues of vandalism and vagrancy highlighted in Perone’s study. More research out of Tallinn will further illuminate the issue, but as congestion pricing and taxation remain politically poisonous issues, it is likely that we’ll see more fare increases before U.S. cities take up any form of the experiment again.