Our survey also shows that less parking leads to less car ownership and less driving. In the buildings we examined without on-site parking, only 38% of households own a car. In buildings with at least one parking space per unit, more than 81% do (see Figure 1). This impact of parking supply on car ownership remains similar even after controlling for transit, walk, and bicycle accessibility, household income, and other characteristics.
Figure 1. More parking leads to more car ownership in San Francisco
In short, the more parking that is provided, the more that residents drive, and the less they travel by public transit, walking, and cycling. Non-work trips are affected even more than commute trips — possibly because commute trips are more constrained by the availability of parking (or transit) at the workplace, and possibly because many commutes are of a distance that precludes walking or biking. In contrast, when people make non-commuting trips, such as to the grocery store, they have more choice in potential destinations (e.g., there might be two grocery stores a short distance away), and in that situation, it is the availability or not of residential parking that exerts greater influence on mode choice.
In recent years, San Francisco has made significant changes to its parking policy. Instead of requiring new buildings to provide a minimum amount of parking — typically one parking space per residential unit — San Francisco now caps the amount of parking in transit-friendly neighborhoods, often at one space for every two or four units. Our survey demonstrates how these policy changes have directly led to reductions in car ownership and driving.
Impacts on Employment
Less parking leads to less driving. That’s certainly good for the environment, but it’s natural to wonder if reducing parking implies some costs in the form of limited employment opportunities. Many jobs are inaccessible by public transit; car access could improve the chances of getting and keeping a job, and a building that discouraged residents from owning a car — through a lack of parking — could reduce those chances.
Our survey, however, showed no evidence that this trade-off exists. More on-site residential parking has no detectable impact on commute length or employment mobility (a measure of worker flexibility to change jobs). Greater transit accessibility, in contrast, has a moderately positive effect on these variables — better transit means shorter commutes and slightly more job turnover. And neither transit accessibility nor parking ratios affect the probability of a household’s members being employed full-time, possibly because of the strong economy and minimal unemployment in San Francisco at the time of the survey in 2019.
Researchers have long suspected that the built environment directly affects people’s travel decisions, but this intuition, while backed by a lot of suggestive evidence, has always been haunted by the fact that people choose where they live. Our research, by exploiting the random assignment of a broad variety of people into housing units and neighborhoods, gets us much closer to seeing that the built environment really does cause people to change their travel behavior. Even within San Francisco, a more walkable, bikeable, and transit-accessible city than most other places in the U.S., transit accessibility substantially affects car ownership and travel behavior. This suggests that even more substantial household responses to increased bus frequency, for example, might be expected in places where transit service is minimal at present.
We find that a building’s parking provision has an even stronger effect in shaping transportation outcomes. Because households may have several parking options — in the building, on the street, in a public garage, or in a space rented in a nearby building — one might surmise that a building’s on-site parking supply would have only a small impact on car ownership. Perhaps surprisingly, however, we show that buildings with at least one parking space per unit (as required by zoning codes in most U.S. cities, and in San Francisco until circa 2010) have more than twice the car ownership rate of buildings with no parking. If parking is provided on-site for free or at a reduced price (typically, $100 per month), then households take advantage of this amenity. In contrast, households without access to on-site parking are more likely to forgo car ownership altogether, rather than deal with the hassle of street parking or walking to a nearby garage.
Cities that want to encourage transit use and walking should consider reducing the quantity of residential parking as well as plan for alternatives to the private car. Transit accessibility evolves over decades, and a concerted effort to improve local infrastructure requires large amounts of public funding. Parking ratios, in contrast, require only regulatory changes to zoning codes: removing minimum requirements from zoning codes and possibly replacing them with a maximum. Such zoning reforms could also yield other benefits, including lowering housing costs, making more land available for new housing and commercial development, and reducing motor vehicle trips and their associated harms. Moreover, reducing space dedicated to parking appears to produce benefits without hurting employment.