Effects of Parking Provision on Automobile Use in Cities Inferring Causality

Many cities include minimum parking requirements in their zoning codes and provide ample parking for public use. However, parking is costly to provide and encourages automobile use, according to many site-specific studies. At the city scale, higher automobile use is linked to traffic congestion, environmental degradation, and negative health and safety impacts, but there is a lack of compelling, consolidated evidence that large-scale parking increases cause automobile use to rise. In this study, the Bradford Hill criteria, adopted from the field of epidemiology, were applied to determine whether increases in parking should be considered a likely cause of citywide increases in automobile use. Prior research and original data from nine U.S. cities dating to 1960 were relied on. It was found that an increase in parking provision from 0.1 to 0.5 parking space per person was associated with an increase in automobile mode share of roughly 30 percentage points. It was also demonstrated that a majority of the Bradford Hill criteria could be satisfied by using the available data; this finding offers compelling evidence that parking provision is a cause of citywide automobile use. Given the costs associated with parking and its apparent effects on automobile use, these findings warrant policies to restrict and reduce parking capacity in cities.