In March, when a cloud of particle pollution settled across Western Europe, Paris took a radical approach. The Ile-de-France region introduced alternate driving days (odd-number plates one day; evens the next) and eliminated fares on local trams, buses, trains and subways.

Traffic dropped by nearly 20 percent in Paris; congestion on the PÃ©riphÃ©rique ring road fell by 30 percent at rush hour; large-particle pollution fell by 6 percent. Measured by the impact on the roadways, the emergency measures worked as intended.

And on the rails? Unfortunately, the open-gate policy meant that the transportation authority didn’t count how many travelers boarded trains, subways, buses and trams during the fare-free days. The city performed a huge experiment in transportation policy, and nobody bothered to watch.

It doesn’t matter much in context. We can’t expect the traffic-choked French capital to make a habit of such initiatives. Alternate driving days are an intolerable hassle for car-dependent commuters; lost fares and the provision of supplementary service to the tune of 600,000 seats on the MÃ©tro, the tramway and suburban rail system cost the region nearly $3.5 million per day. Fares cover nearly half the operating costs of the RATP, the state-owned transit operator, so eliminating them would put a tremendous hole in the annual budget.

And yet, Paris would have been a valuable case study. The consequences of eliminating transit fares remain surprisingly obscure. Can a fare-free policy transform a regional transportation picture? Can it pay for itself? Or is it merely a publicity gimmick that inflicts needless financial woes on local transit agencies?

Many people reject the idea out of hand, saying free rides are a problem, not a solution. But “free” transit, of course, is only as free as public libraries, parks and highways, which is to say that the financial burden is merely transferred from individual riders to a municipal general fund, a sales tax or local businesses and property owners. A free ride policy represents the culmination of a long shift from thinking of transit as a business sector — one that was quite profitable in its heyday — to considering it an indispensable public service.