In our current times, it seems that there is a story virtually every day about the confrontations between American citizens and the police. In Policing the Open Road: How Cars Transformed American Freedom, Sarah Seo speaks not only to the basic history of the automobile, the expanded role of law enforcement triggered by the automobile, and the increasing frequency of contact between officers and the public, but also to the role that the automobile played in the evolving tasks of the police and the implementation of police policy. Not only is Seo's book revolutionary in its examination of automobiles and how they changed Americans’ relationship with the government, it also demonstrates how cars were actually the beginning of the redefinition and, arguably, downfall, of the protections afforded by the Fourth Amendment's search and seizure power, given the interest that police had in searching the back seats and opening the trunks of people's cars to find evidence of crimes.

Cars allowed people to become more mobile, necessitating changes in the law, both to regulate the new machines and people's behavior in them. Seo begins her book by detailing the very basic questions that were now quite important with the increasing use of the automobile such as: How fast was too fast to drive? How would drivers know whose turn it was to cross an intersection? How should cars and pedestrians interact? What color should a stop sign be? Because these questions had few answers in the early days of motorized travel, there were numerous accidents, involving both cars and pedestrians, and many of them were fatal.

One important unintended consequence of the automobile also began to rear its ugly head shortly after the automobile's invention: criminals could now escape from the scene of the crime much more quickly than they could have on horseback. Cars made it much easier for criminals, as well as their contraband, to cross not only city boundaries, but also state lines. As Seo explains, prior to Prohibition, cars, and modern policing, police searches rarely implicated the Fourth Amendment. Before the twentieth century, Seo writes, “the only real recourse for a violation of the Fourth Amendment was a civil action” and very little damages were available in such a situation (117). However, the interaction between Prohibition and the automobile brought the Fourth Amendment into focus for what was really the first time; because “cars provided both the getaway and a cover for hiding things, officers not only had insufficient time to get a warrant but also usually had only a mere suspicion, short of probable cause or knowledge” required for such a warrant (115–16). By the time they applied to a judge, the suspect would be long gone.

The Supreme Court had begun to confront “the Exclusionary Rule” in 1914 with Weeks v. United States, but only some jurisdictions chose to adopt the rule as part of their state procedure. Once Prohibition, the widespread use of the automobile, and the rule of exclusion converged, this confluence raised one of the “most contentious questions in twentieth-century criminal procedure: when did the Fourth Amendment require a warrant to stop and search a car?” (119). The classification of a car lay at the crux of the issue, as cars were certainly private property, but were unlike other forms of searchable stationary property. Was the car simply a mode of transportation, or was it a private space, akin to a house or a dwelling? Was the search of it different than the search of a saddlebag on a horse or a compartment of a boat, other forms of property that were mobile and on which a suspect might escape? As the court began to confront more and more automobile cases, the public/private distinction began to trend toward the public interest, meaning that what constituted an “unreasonable” search and seizure was construed to give more and more leeway to the police. Finally, in Carroll v. United States (1925), the court announced that a “warrantless search and seizure was lawful if the ‘officer shall have reasonable or probable cause for believing that the automobile which he stops and seizes has contraband,’” a decision that demonstrates the court's cognizance of the complications of automobiles and Prohibition (emphasis in original) (137–38).

This newfound grant of discretion to the police would only continue to expand as the decades passed. Arbitrary policing began to explode during the Cold War, as it was now a “prevalent method of enforcing social order…at a time when people were obsessively comparing democracy with totalitarianism” (158). Seo details how automobile stops became much more common and expanded into searches of persons and their effects, not just back seats and trunks. By the time the court confronted Monroe v. Pape and Mapp v. Ohio in the late 1950s and early 1960s, Fourth Amendment searches had moved from having a strict textualist interpretation that called for a warrant in order to perform a search, to having one of reasonableness: officers could now take action if they believed “the situation at hand called for it” (138). Predictably, this had disastrous results for civil liberties, particularly for minorities, as Seo details in Chapters Four and Five of the book.

There is much to like about Seo's book, although for the casual reader, the earlier chapters are the more enjoyable, mostly because of the focus on the evolution of basic driving laws and the uniformity that would become necessary in order to properly regulate the new technology. Yellow stop signs would, unfortunately, be retired as automobiles became part of the normal makeup of American life, with uniform standards and expanded police discretion in their place. But in addition, the Fourth Amendment would be redefined, and citizens’ contacts with police would become more and more fraught.