As America's oldest roads reach the end of their useful life, traffic congestion continues to worsen, and the nation's primary federal transportation financing vehicle (the Highway Trust Fund) approaches insolvency, U.S. policymakers must consider ways to incentivize alternative modes of transportation such as public transportation. In addition to providing accessibility, public transportation also plays a role in reducing carbon emissions and easing traffic congestion---two very serious concerns for American metropolitan areas. In the next few decades, Americans will almost certainly have to drive less and rely more on public transit for the sake of our environment and infrastructure. It is essential to identify policies that incentivize these changes among Americans on all points of the economic spectrum. This paper examines the rates of public transportation use among American workers and seeks to identify the key factors explaining transit use and how those factors vary across income levels. Prior research has investigated both supply-side and demand-side factors explaining Americans' transit use, but few studies, if any, have examined to what extent these explanatory factors vary at different levels of income. Although public transit is frequently cited as a textbook example of an inferior good, evidence suggests that transit is actually a normal good at the highest levels of income; indeed, the data show low- and high-income commuters already use transit at comparable rates. I hypothesize that the transit-income relationship is non-linear and that the effect of income on transit is positive at higher levels of income. Drawing on data from the 2009 American Community Survey, the American Public Transportation Association, and the Texas A&M Transportation Institute, I construct a national snapshot of American commuters that captures levels and costs of transit service, levels of traffic congestion, and automobile access, some of the key explanatory factors identified in prior research. Utilizing logit regression models, I investigate to what extent personal and external factors affect the probability of an individual's transit use at varying levels of income. This thesis aims to further discussions of public transit in transportation planning and to inform policy makers of the potential distributional and social equity implications of future transportation policies.
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