Abstract:The research presented here explores the role of social networks in transportation mode choice. Social networks serve as a foundation for multiple social processes, such as cooperation, resource sharing and social influence. The focus of this study is social influence, whereby the knowledge, behaviors and/or opinions of one individual affect those of another. Through an online survey of students at the University of California, Davis, I collected information about students’ transportation decisions for campus travel and their social networks. For each participant as an ego, I gathered information about their ego-network including up to five alters or contacts. The ego-networks, representing a subset of the social network of each respondent, were analyzed for similarities in transportation mode choice. Each respondent’s mode choice was also compared to the mode use of geographic neighbors. Individuals tend to make transportation choices similar to both those to whom they are socially connected and those with whom they share geographic proximity. The first paper in this study presents evidence that although similarities in transportation mode choice occur both for socially connected individuals as well as geographic neighbors, social and neighborhood effects are not the same. To account for the possibility that similar commute characteristics simultaneously affect socially connected individuals, in the second paper presented here, I use an instrumental variables approach and find evidence of social influence even when accounting for the commute characteristics of both the alters and ego in ego-networks. In the third paper, I demonstrate that social influence does not affect all individuals equally; in particular social influence related to biking has a smaller effect on those with longer or very short commute distances. This study improves our understanding of social influences in travel behavior and the results may inform sustainable transportation programs and policies.