Widespread automobile use has left a distinctive mark on urban transportation in the United States, where growth has been tailored to automobiles. Although cars enable people to travel farther than at any point in human history, evidence of their costs has grown. In particular, the environmental impacts of automobiles are extensive, including air and water pollution, greenhouse gas emissions, and reductions in urban transportation ecoefficiency (TE). Overall, TE is the environmental impact per unit of travel; as a concept it can capture differential impacts of different transportation modes and travel distances. There are large variations in TE between metropolitan areas, and it is important to investigate factors that influence it to determine what measures can reduce the environmental impact of transportation. This dissertation describes the concept of TE in depth, arguing that it captures the systemic environmental impacts of transportation and is ideal for macro-level transportation studies. A TE measure using readily available data is then proposed, with four components: 1) percent of commuters driving to work; 2) percent of commuters taking public transit; 3) percent of commuters walking or riding a bicycle; and 4) population density. A confirmatory factor analysis and discussion of TE trends demonstrate that these components are useful for measuring TE and consistent in their relationships over time. Using this measure the trend for TE is downward – most U.S. metropolitan areas have seen TE decline over the last 30 years.Next, a macro-level analysis of institutional and structural predictors of TE is presented. Specifically, a general linear regression is used to predict TE change from 1980 to 2008, for a sample of 225 Metropolitan Statistical Areas (MSAs). Since the trend in TE is downward, a positive relationship indicates a slower decline in most cases. Results point to the following conclusions: * A New Political Culture (NPC), measured by professional occupations, education, and income, reduces the decline in TE, although professional status could reverse this effect. * High and rising incomes interact to further reduce TE declines, supporting the influence of the NPC’s economic side. * State-mandated urban growth management also reduces TE declines, demonstrating the beneficial effects of comprehensive planning. This effect is enhanced by community wealth.* State government fragmentation creates larger TE declines, which provides support for the role of coordinated governance in transportation improvements.* Racial diversity (% African American) has a quadratic influence, which could be because racial tolerance is beneficial for transportation, but racial conflict has the opposite effect.* Positive interactions between % African American and both income variables (real per capita income and % change in real per capita income) support the argument that racial economic threat or conflict creates larger declines in TE. * These processes vary by census region, which implies the influence of an exogenous regional variable, such as cultural values or cultural diffusion.Overall, this analysis demonstrates that macro-level social processes have a significant impact on urban TE, including race, policy and governance, and the presence of a new political culture.