By Chris McCahill
More studies over the years have shown us that the price and availability of parking has a strong influence on people’s travel choices. A ten-year-old study from New York, for instance, called attention to the influence of parking availability on people’s decision to drive to work. Several years later, I led a study connecting long-term parking growth to citywide increases in car commuting. Now a new study by a cohort of researchers across North America, including myself, makes that connection even clearer by drawing a direct line from residential parking ratios to household VMT.
The new study—led by Kristina Currans at the University of Arizona, Gabriella Abou-Zeid at ICF, and others—focuses on parking and travel behavior across Los Angeles County using data from the 2017 National Household Travel Survey paired with estimates of parking supply at each location. After controlling for factors like place type and household size, it found that vehicle ownership rates are 14 percent higher for households with more than one available parking space per unit, compared to those with constrained parking. It then found that vehicle ownership translates into travel demand: “For every additional vehicle in the average household, we estimate the household travels approximately 17 more miles of total [daily] VMT and 3 more miles in home-based work VMT.”
Because of this two-step modeling approach, the study presents several scenarios to help illustrate the link between parking availability and commute travel, depicted below. The baseline scenario is an average suburban household with no information about the available parking. Households with more than one parking space available tend to drive six percent more for commuting and those with more constrained parking tend to drive 21 percent less. That pattern holds true in different place types, even as the average VMT gradually decreases in more urban neighborhoods. A household with unconstrained parking in the urban core drives around half as much for commuting as the average suburban household, but that number drops to 29 percent if there are parking constraints—a 40 percent difference. These differences in work travel also translate to differences in total household VMT, although the overall effects are smaller.
The study points to the importance of these findings in travel demand modeling, which often ignores the effects of parking availability. It explains:
For too long, parking has been relegated to proxies in travel demand research. To understand the effect of vehicle travel through land use, we must incorporate off-street parking supply into analyses exploring the influence of the built environment. Without explicit controls—and an understanding of how to effectively consider those controls in analysis and practice—studies that do not incorporate parking ignore a major element of the built environment. This leaves a critical gap in cases where parking supply does not align well with conventional expectations, such as with developers building large underground parking garages in urban centers and thus ignoring a potential policy lever.
The implications for parking policy and transportation demand management, alluded to above, are also important. As the study notes, there are issues of self-selection to consider—people choose to live in places that meet their lifestyle expectations—yet there are good reasons to consider these relationships when building or encouraging new development that satisfies and maybe even promotes living with less driving. As more transportation agencies like those in California, Minnesota, and Colorado focus on curbing or mitigating travel demand, local land use and parking policies will be a critical part of the equation.