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A new study finds a "clear relationship" between a guaranteed spot at home and commuting by car.

For 30 years now Manhattan has imposed parking maximums on its core — a rule that limits the number of spots residential developers can create. The idea behind parking maximums is that by removing an incentive to own an automobile, cities will promote the use of more sustainable forms of transportation. For all its soundness of theory, though, the effects of residential parking on travel behavior hasn't received its due attention from the empirical research world.

Planning professor Rachel Weinberger of the University of Pennsylvania addresses this gap in the literature in an upcoming issue of Transport Policy. To demonstrate how parking maximums might deter driving, Weinberger studies how parking minimums influence commuting behavior in three outer New York City boroughs: Brooklyn, Queens, and the Bronx. These areas served as a logical testing ground because residents have good transit access to Manhattan while off-street parking remains quite scarce.

Weinberger's analysis proceeded in two steps. First she estimated the chances that any property in these boroughs had a private parking spot by gathering as much data on off-street parking as she could. Her resources included city data kept on residential garage space for buildings with four or more families, and aerial images from Google Earth (later confirmed by field tests) for smaller buildings. She concluded that Brooklyn had the fewest buildings with on-site parking (57 percent), following by the Bronx (71 percent), then Queens (79 percent).

Weinberger then paired this information with Census data on the number of people who commute into the Manhattan core by car from each outer borough neighborhood. She restricted her Manhattan destinations to those parts of the core within a quarter-mile of a transit station, which ends up being most of the borough below 96th street, to ensure that travelers had reasonable access to their work by some means other than driving.

The statistical models that emerged from all this information showed that even after controlling for factors like income, housing stock, and transit accessibility, outer borough neighborhoods with higher levels of private, on-site parking had more residents driving to work in Manhattan. In other words, Weinberger concludes, a guaranteed parking spot "makes use of the automobile a more attractive option":

The research shows a clear relationship between guaranteed parking at home and the greater propensity to use the automobile for journey to work trips even between origin and destinations pairs that are reasonably well and very well served by transit. Because journey to work trips to the downtown are typically well served by transit, we infer from this finding that trips for other purposes from these areas of higher on-site, off-street parking are also made disproportionately by car.

To Weinberger, the policy implications of this finding are clear: minimum parking requirements encourage driving even in places where other forms of transportation would otherwise suffice. On the flip side, the results also suggest the effectiveness of parking maximums. Remove the incentive to drive, and alternative transport modes experience a relative gain in attractiveness:

Cities have long had the intuition that reducing parking requirements, indeed implementing parking maximums, will have the effect of reducing auto use. ... Until now, that intuition has not been verified. Armed with new knowledge that minimum parking requirements will lead to additional driving, cities, particularly those in non-attainment for air quality standards and those seeking to increase use of transport modes that are more energy and space efficient than private automobiles, should consider this information when crafting their residential parking policies. 

Top image credit: l i g h t p o e t / Shutterstock.com

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