By Chris McCahill
Public transit is often viewed by planners and transportation officials as a key strategy for reducing car dependency, easing traffic congestion, and lowering emissions. Every person who opts for transit over driving helps reduce the total miles driven in private vehicles. According to a large national study, however, good transit has a ripple effect on land use and travel behavior. For every mile not driven by transit riders, transit accounts for another six to nine miles not driven among the larger population.
This latest study from the University of Utah and MIT relied on a massive collection of travel surveys spanning more than 80,000 households across 28 regions of the U.S. This is more than three times the number of households in the 2017 National Household Travel Survey, and ten times the number in the most recent 2022 survey.
The study looked at how factors like transit access, household makeup and income, and fuel prices influence transit use and vehicle miles of travel (VMT). The researchers measured transit access as a percentage of regional jobs that can be reached within 30 minutes—a method we describe in our guide, Measuring Accessibility.
Their models differentiated between direct and indirect impacts. Direct impacts refer to when someone switches from driving to taking transit, thereby driving fewer miles. The researchers note, however, “the most noticeable effect of public transportation is not caused by people giving up their cars in favor of riding the bus or subway, but by the development of an infrastructure that can accommodate such riders.” Those are the indirect impacts.
In most smaller regions, the indirect impacts are six times larger than direct impacts. They are around nine times larger in “transit-rich” areas like Boston and Seattle. The researchers explain with an example:
Let’s take a region with 1,000,000 annual transportation passenger miles as an example to see what a multiplier of 6.59 really means. Applying a mode shift factor of 0.329, we can estimate that these 1 million passenger miles would have otherwise accounted for approximately 329,000 personal vehicle miles (1,000,000 * 0.329 = 329,000). By applying the transit multiplier of 6.59, we can utilize this value of 329,000 VMT to estimate the broader community’s reduction in VMT by accounting for land use efficiency […] So, taken together, the entire reduction in miles traveled, including transit passengers and the broader community, would thus equal about 2,168,110 miles.
This research underscores a powerful insight: transit isn’t just about moving people; it’s about transforming communities. As planners and policymakers look ahead, recognizing these broader impacts could help bolster support for much-needed investments.