By Sarah A. Seo

Is a mobile home more like an automobile or a house? This was the key question that the justices of the US Supreme Court had to determine in California v. Carney, a 1985 case about the warrantless search of a mobile home parked in a lot in downtown San Diego. An ordinary person’s answer to the question would be “both.” But the law often relies on mutually exclusive categories. Whether the mobile home was an automobile or a house mattered because under the Fourth Amendment, law enforcement needed a warrant to search a house but not to search a car. If the justices were to decide that the mobile home was a house, then—because the DEA agents in the case conducted a warrantless search—the marijuana that they found could not be used as evidence against the defendant Carney. Without evidence, the prosecution no longer had a case.

Oral argument in Carney began abstractly, almost philosophically. What makes a car a car? The government maintained that a car’s essence was its mobility, the capability for “self-locomotion.” The defense argued that even with wheels, the more that a car takes on the attributes of a house—like having window drapes, upholstered furniture, a kitchen—then it becomes one. After the lawyers staked their positions, the justices peppered them with hypothetical questions that tested the limits of their competing definitions. What if the mobile home sat in a trailer park and was hooked up to water and electricity? What if a mobile home, even one with drapes and furniture, was traveling on a public highway?

Ultimately, a majority on the court decided that a mobile home, so long as it had wheels, was a car for Fourth Amendment purposes. But the opinion rested its conclusion not just on “ready mobility.” The more fundamental reason was that motorists had “reduced expectations of privacy” because of “the pervasive regulation of vehicles capable of traveling on the public highways.” In other words, what made a car a car was government regulation.

California v. Carney offers several points of consideration for urban historians. For one thing, it shows how urban spaces can give meaning to an object, like a car, and vice versa, how objects can profoundly shape the complex composition of urban spaces. As I show in Policing the Open Road, the mass production of cars immediately created chaos and disorder, especially in downtown commercial areas in towns and cities throughout the country. Rather than “moving the city,” so to speak, too many cars on streets originally meant for a few horse-drawn carriages and mostly pedestrians stopped traffic. If not at a standstill, then these dangerous machines—as automobiles were often called then—moved too fast, resulting in horrific accidents and fatalities that frequently involved children who were accustomed to playing on the streets. To protect the public’s safety and to get traffic moving again, local governments regulated just about every aspect of operating a motor vehicle, including license and registration requirements, minimum safety equipment, speed limits, rights of way, and more.

From the earliest years of mass-produced cars, courts defined cars in reference to the reality that they were extensively regulated, so much more than houses or any other property that individuals owned. As a Philadelphia court explained in the 1923 case Commonwealth v. Street, cars were “peculiarly the subject of government control.” “Were this not so,” the opinion continued, “modern life and society would be hopelessly imperiled.” And this was particularly so in more crowded, urban areas, where traffic laws that set forth uniform, consistent, and safe driving norms were especially necessary.

The consequences of regulation were vast. According to Commonwealth v. Street, “the right to stop and search an automobile for liquor”—that would be marijuana in Carney—“is no different from the right to stop and search an automobile … to discover whether or not the operator of the vehicle has in his possession the license card provided by the automobile statutes of the State.” The court held that the state’s authority to regulate the traffic of motor vehicles also included the police’s power to search them for criminal evidence without the permission of a neutral judge in the form of a warrant. To put it another way, the Fourth Amendment’s protections that applied to houses and “effects”—which means any personal property—did not apply to the automobile. Cars were defined out of the Fourth Amendment’s ambit.

Fourth Amendment car cases like California v. Carney and Commonwealth v. Street also highlight how people and movement were policed in the twentieth century. They were policed in a mode of transportation that ordinary people often think of as private spaces, in contrast to the “public” transit of buses, subways, and trains. But the legal status of cars as private spaces—as private property even—was doubtful. As the Street court explained, even though automobiles were privately owned like houses, they were “in an entirely different position.” Or, as the Carney opinion recited the oft-quoted phrase, drivers had “reduced expectations of privacy” in their cars. It was the mass mobilization of individualized, “privately owned” transport that made cars so dangerous, which consequently and paradoxically subjected motorists to much more regulation, and thus also policing, than they had ever before experienced when traveling in carriages, buses, trains, trolleys, or ships.

As much as courts throughout the twentieth century sought to differentiate cars from houses, the automobile straddled the public/private divide in American life. Notwithstanding how much the Fourth Amendment accommodated the expansion of law enforcement’s discretionary power to police cars and their occupants, twentieth-century culture instilled motorists’ sense of autonomy, self-determination, and privacy. Policing the Open Road examines this tension between public order and private rights to explain the emergence of a new definition of freedom in American constitutional law—a legal resolution that has also contributed to the disproportionate incarceration of people of color today. To understand how we got to this present moment, I began by examining the history of policing cars; after all, the first moment of many people’s encounter with the police begins with a traffic stop, a fact of life in a car culture and society. What I discovered was an enduring contestation over the meaning and status of the automobile, which expanded my conceptualization of space and offered new perspectives on the reach of state power in the twentieth century.

Sarah A. Seo is Professor of Law at the University of Iowa College of Law. She is the author of Policing the Open Road: How Cars Transformed American Freedom, published by Harvard University Press in 2019.

Featured image (at top): Interior of Freedom Riders’ Bus, with view through window of line of 6 police cars and soldiers lining pavement, 1961, Prints and Photographs Division, Library of Congress