Amy Auchincloss, an epidemiologist at Drexel University, believes that to make cities more livable, we need to cut down on the number of cars on the roads.

“High private automobile use has devastating consequences for health and fosters inequitable access to amenities,” she says in an email. “Reliance on automobiles adds to the costs of living in cities and makes it harder to improve infrastructure for public transit and active travel.”

While this kind of talk can drive dedicated drivers nuts, urban planners often agree with Auchincloss. They've tried various strategies for discouraging car use in cities around the world, including expanding railways and building bike lanes. One of the most common strategies is hiking up the cost of on- and off-street parking—an intuitive approach. Its effectiveness has been difficult to verify, though, because so many different factors contribute to car volume in any given city.

Among large cities, at least, higher parking costs are positively correlated with the average number of miles each city dweller spends on public transit.

To see if a case actually should be made for more expensive parking, Auchincloss and colleagues at Drexel, the University of New Hampshire, and Nelson/Nygaard Consulting Associates recently decided to take a broader-scale approach. By first crunching some numbers from a 2009 survey of public parking agencies in 107 U.S. cities, and then comparing them with those cities’ population densities and public transportation data, they found that, among large cities, at least, higher parking costs are positively correlated with the average number of miles each city dweller spends on public transit.

While this correlation isn’t surprising, identifying it remains an important step forward in city planning, the researchers say in their study published in Public Works Management & Policy. Cities tend to lack this type of big-picture perspective when it comes parking and transportation decision-making.

“In the case of parking, we found that multiple agencies (and in some cases private entities) were involved with data on public parking and public works officers—who play a key role in developing transportation policies—knew very little about land use development/zoning for parking ... or even revenues generated from parking," Auchincloss says. “[Our study] provides baseline data that can be used by practitioners/planners in public works departments when considering local parking policies.”

Auchincloss refers to the study as “very exploratory,” because there are, in the study’s words, “myriad interrelated factors not included in these analyses” (such as state-level land use policies, access to public transportation, and public transportation’s varying costs) that prevent the researchers from being able to say more expensive parking actually causes more people to ditch their cars and hop on the subway. The study is only an early step forward in understanding parking's role in shaping urban transportation—but one that Auchincloss nonetheless feels sends an important message to urban planners looking to do what's best for their cities.

“Most of us will pick the most convenient, fastest, cheapest way to get around," she says. "It is reasonable to expect that if public transit (or active travel) ranks higher on those dimensions than driving—then more people will put aside their car (or not want to own a car)."