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U.S. residents travel in cars for the vast majority of their trips. Yet car travel imposes costs on the individual (in terms of out-of-pocket costs) and society (via air pollution, congestion, and collision rates). Urban planners, policymakers, and elected officials have enacted many policies to subsidize and promote shared-vehicle travel – including via public transit – to mitigate these costs. Yet despite the billions of public dollars poured into public transportation, most shared travel in the U.S. occurs in private cars. And by extending mobility to people with limited car access, informal automobile sharing – with household members, friends, coworkers, and strangers – also offers benefits to disadvantaged travelers. But due to data limitations and policy emphases on public forms of shared travel, few researchers have systematically examined the relationship between transportation disadvantage and informal sharing.In this dissertation, I use mixed methods to answer different aspects of a single question: What utility does informal vehicle-sharing offer U.S. travelers? In the first essay, I use quantitative methods to explore the factors that determine whether a traveler chooses to share. In particular, I emphasize how disadvantage (in the form of medical conditions, poverty, and vehicle access) influences informal automobile sharing. Using data from the 2017 National Household Travel Survey (NHTS), I model automobile sharing as an expression of rational choice and thus a proxy for the utility it offers travelers. I find that while transportation disadvantage is associated with certain sharing behaviors (including borrowing cars and receiving rides from people living in other households), trip purposes – particularly non-work-related ones – best predict the likelihood of sharing a private vehicle.In the latter two essays, I analyze data from the Craigslist rideshare board to examine the opportunities and challenges people face in ridesharing with strangers. In the second essay, I use mixed methods to analyze web-scraped data. I examine the physical qualities of trips desired and offered on Craigslist and how they varied across California regions. I also measure the frequency and types of compensation that posters mentioned. In the third essay, I use qualitative methods to analyze information from surveys of and interviews with people who posted on the Craigslist rideshare board. I evaluate how often they successfully rideshared and how they balanced the risks and rewards of interacting with strangers.Findings from the three essays highlight the potential of informal vehicle-sharing to address social and environmental challenges in the U.S. Millions of empty seats fill streets and highways every day, while many disadvantaged people struggle to meet their daily travel needs. Based on my findings, I recommend policies that provide automobile-based assistance – such as subsidized carshare programs – to low-income families. To facilitate ridesharing between strangers, I recommend that public agencies create digital applications to help people match with other travelers. I also recommend that agencies consider pricing road travel by distance, to make non-sharing – and particularly driving alone across long distances – costlier. Doing so will help increase opportunities to share for all travelers.
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