Earlier this week, with the city shrouded in unusually toxic smog, Paris tried a tactic unheard-of in U.S. cities to at least temporarily clear the air. It announced plans to ban tens of thousands of cars from city streets — the even-numbered license plates on Monday, the odds on Tuesday. Hybrids, electric cars and carpools were given a reprieve. But the abrupt policy otherwise promised to massively disrupt daily commutes, marking the first such auto rationing on Parisian streets since 1997.
By the end of the day on Monday — after some 4,000 cranky motorists had been ticketed — the idea was called off. The weather was already starting to shift (taking with it any enthusiasm for drastic solutions). But the abbreviated experiment brought new attention to an idea that's already commonplace in other parts of the world. Beijing famously kicked cars off the road during the 2008 Olympics with a similar odd/even system. The Chinese city still bans cars one day a week, in an effort to both ration road space and curb pollution. Mexico City does the same.
From an environmental standpoint, there are at least two ways to try to rein in pollution from vehicles: We can either improve the technology itself (getting cleaner, more efficient cars on the road), or we can try to reduce how much people use it. Unfortunately, evidence suggests that attempts at the second strategy often undercut the first one.
Research out of Beijing has found that the ongoing one-day-a-week ban has reduced particulate matter there by about nine percent. But a study out of Mexico City found no positive environmental benefits from the regular ban there, for a fascinating reason. People really like to drive, and they'll come up with some seriously inventive ways around restrictions. In Mexico City, it appears that many people bought cheap, used second cars (you've got your odd car and your even car) to get around the license plate rule. In effect, it appears as if the ban caused an increase in the total number of cars on Mexico City's roads, tilting the makeup of the entire fleet toward less fuel-efficient vehicles.
Meanwhile, there was no evidence in Mexico City that more people were taking public transit because of the driving ban.
That the ban caused such perverse incentives underscores the difficulty of making the idea work. Paris tried to sweeten the alternatives to driving by simultaneously making public transit and bikeshare free. It also allowed free street parking for banned cars that weren't allowed in circulation. But the free public transportation alone reportedly cost the city about 4 million Euros. Clearly, the strategy isn't financially or politically sustainable outside of rare emergencies. And yet, reducing pollution (while also easing congestion) is an ongoing policy goal for many cities outside of once-in-a-decade smog days.
So what's a better idea? Incentives to get people into cleaner cars would no doubt be gentler than disincentives to driving as much. It's also hard to ask people not to drive without giving them good alternatives, whether that means improving public transit or expanding telework. Let a guy work from home, and that takes a car off the road just the same as a ban would. There's also the option that we might increase the cost of driving rather than outright ban it, as central London and other European cities have done with congestion pricing. That strategy comes with its own equity challenges. None of these ideas is obviously easy. But the best solution will keep in mind inventive human behavior.