In recent years, political scientists have begun to pay more attention to both political geography and political polarization. In The Road to Inequality, Clayton Nall brings these two threads together in a compelling analysis of the long-term impacts of the federal highway system on American cities. At its core, Nall’s argument posits that federal highways are an exogenous force that altered the political dynamics of metropolitan areas, largely by enabling suburban areas surrounding central cities to become overwhelmingly Republican. Specifically, Nall argues that highways had both a catalyzing effect, speeding up the rate at which Republicans or likely Republicans moved to the suburbs, and a filtering effect, by only allowing some urban residents (notably those who could afford cars) to move to the suburbs.

The increasingly Republican nature of the suburbs has had long-term political effects, including on equality. Nall argues those long-term effects emerged due to transportation policy, which has become increasingly partisan in recent years. While highway spending tends to be supported by both Republicans and Democrats, Republicans are much more wary of spending money on mass transit options within central cities. Further, a great deal of transportation spending is locally determined, with local governments having significant input into the ways in which federal dollars are spent. Additionally, state and regional institutions are biased towards suburban representation. As a result, suburban governments are able to exert a surprising degree of influence over metropolitan transportation spending. Over time, this geographic polarization around transit policy has real impacts, as central cities struggle to provide needed transportation infrastructure to citizens. Less affluent citizens, who rely on public transit, are especially impacted by the results of this geographic polarization.

Nall builds this argument in two basic parts. In the first half of the book, he explores how highways have enabled partisan sorting. In building this argument, he makes exceptional use of innovative data and methods. For instance, he draws from travel time data collected by Rand McNally in order to explore how highways allowed suburban dwellers to commute more easily into central cities. He also makes excellent use of real estate advertising, historical surveys such as the Youth-Parent Socialization Panel Study, and other sources to carefully detail how highways not only encouraged suburbanization but specifically allowed suburbs to become heavily Republican. Then, in the second half of the book, Nall draws from a number of surveys across time to demonstrate that individuals’ position on transportation policy is not merely self-interested and place based but actually motivated by partisanship. He turns to the General Social Survey and the Roper Center for Public Opinion Research data in particular, in order to illustrate how transportation policy is a partisan issue. Finally, he also relies on original survey work to further underscore his findings.

The author works diligently to replicate his findings across different data sources. Further, his description of the data and his methods are always clear and insightful. He is similarly measured in his conclusions, expressing wariness about geographic sorting producing “echo chambers,” which in turn fuel greater polarization. Finally, Nall ends his book by outlining how this geographic sorting has real impacts on the nature of transportation policy, which in turn impacts a whole range of other political issues.

In developing his argument, Nall makes a number of important contributions to the urban politics, public policy, and American political development literatures. First, the argument is an exceptional analysis of path dependence. The author takes the American highway system as an exogenous event that is introduced to American cities. From this originating event, he meticulously traces out the demographic and political changes that resulted from American highway policy. The book is a model for how to take a large political event and carefully measure its various intended and unintended political effects. Of course, path-dependent analyses are by no means new. However, this text is an exceptional illustration of how to do this work: Nall uses a range of data sources, carefully considers alternative hypotheses, and bounds his work in theoretical as well as temporal scope. By proceeding with such exhaustive care, he is able to wholly demonstrate the importance of the critical policy on which he focuses.

Related to his careful approach, Nall’s work is also exciting for its unique approach to data analysis. The author combines historical research with quantitative techniques. In addition, he draws from a number of unique data sources as well as geographic information system (GIS) methods to push his argument as far as possible. Traditionally, American political development and, to a lesser extent, the urban politics literature draw heavily from qualitative case-study approaches. Of course, such detailed historical case studies have yielded fantastic histories and rich theoretical insights. However, Nall’s work demonstrates the value of supplementing those qualitative approaches with quantitative tools as well. Notably, the author is able to rigorously consider causal relationships as a result of his quantitative research. Furthermore, the blending of quantitative and qualitative approaches allows him to effectively connect more individual-level behavior, such as voting, with the larger structural issues of highway construction. While he does not claim that highways ever caused voters to become Republican, he is able to tell a much richer story about the development of partisanship in American metropolitan regions as a result of the wide array of data from which he draws. In this way, he opens up new possibilities for future American political development research.

In addition, I want to underscore the value of Nall’s interest in the built environment. In recent years, social scientists have paid more and more attention to urban geography’s role in shaping political behavior. Nall’s work empirically underscores that the built environment does play an important, exogenous role in social phenomenon. However, the work is also measured, and it reminds readers that space’s influence over social outcomes is not absolute. Hence, Nall’s scholarship also makes important contributions to the political geography literature.

With that said, the text could use a larger theoretical discussion. The analysis of the American highway system and how it changed American politics draws from James Scott’s (1999) Seeing Like a State. In this work, Scott turns his interest in large infrastructure projects into a rumination on state power. While Nall’s focus is admittedly different, I was left curious regarding his broader understanding of such megaprojects. In some ways, these more abstract considerations move against his careful empirical work, and so I understand why he does not directly address them. I found myself especially interested in these questions since Nall, diverging from Scott, perceives that at least some megaprojects are shaped by local rather than national interests. However, this is a minor issue. In The Road to Inequality, Nall has produced an exceptionally smart and well-researched text that makes real contributions to the urban politics, political history, and public policy literatures.