Traffic stops, usually over minor infractions, are one of the most common ways that people interact with police. The frequency with which they turn deadly, often with impunity for the officers responsible, has made them a major focal point in the effort to combat police brutality.

In one recent case, police officers in Memphis, Tennessee, pulled over 29-year-old Tyre Nichols for “reckless driving.” Over the next several minutes, officers brutally beat, kicked, and pepper-sprayed Nichols while screaming conflicting orders at him. Three days later, Nichols died from his injuries. Investigators have since said they were “unable to substantiate” the claim that he was driving recklessly, and five officers have since been charged in Nichols’s death.

Now, Memphis lawmakers are considering legislation that would ban officers from stopping drivers for certain low-level driving offenses. The bill, which is modeled after a Philadelphia law, attempts to reduce the potential for deadly interactions between the police and Black drivers, who are pulled over more frequently than drivers of other races.

How did we reach the point where traffic stops escalate into police killings? In her 2019 book, Policing the Open Road: How Cars Transformed American Freedom, Sarah Seo, a historian of criminal law and procedure and professor at Columbia Law School, examines how the automotive era upended society, dramatically expanded the power and authority of the police, and altered our society in the process, resulting in the traffic enforcement system we have today. Vox spoke to Seo about the legal, social, and historical forces that shaped our modern, deadly approach to traffic enforcement.

This interview has been edited for length and clarity.

I want to start by asking you about the police killing of Tyre Nichols. These incidents of brutality are, sadly, not unusual in the United States. How did this traffic stop turn so deadly?

The unit that pulled over Mr. Nichols was a specialized crime task force. They are trained and delegated to fight crime, and they use minor traffic stops to begin their investigation. So you have a law enforcement unit that’s trained to investigate some of the more serious violent crimes using traffic enforcement as a starting point. It should be totally separate. Traffic law enforcement is an important government function because traffic safety is important, but that shouldn’t be co-opted by a law enforcement unit that’s not trying to enforce traffic laws.

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How did the advent of the automotive age shape law enforcement in America?

Driving and owning cars became very common pretty quickly in American society. Government officials had no idea why almost everybody seemed to be violating traffic laws. Fatal accidents were really common, especially in the early years, to the point where people had morally righteous anger whenever an accident resulted in death. [In his book, Fighting Traffic: The Dawn of The Motor Age in the American City] historian Peter Norton talks about this huge marketing shift that the automobile industry had to do to overcome the anger at the chaos and destruction that cars produced. It resulted in a lot of accidents, and also just traffic. It was chaos on the streets. So municipalities throughout the country passed an increasing number of traffic laws, and everybody violated them. They tried different ways of getting people to obey them, what I call 19th-century forms of getting people to comply with norms: churches, automobile associations, civic associations, and none of those worked. So they resorted pretty quickly to police law enforcement.

Usually, historians, we emphasize change over time, but this is a constant over time: People get really mad at the people who try to enforce traffic laws. And so given the hostility and aggression that enforcers experienced, what municipalities did was increase their discretionary power, so disobeying an order of traffic police became a misdemeanor. Their authority was increased to manage the difficulty of car drivers. At the same time, car drivers hated to be policed in their cars. They were really irate. A lot of the training of traffic enforcers [involved being told to] exercise discretion.

You write in your book about how the Supreme Court dealt with new, difficult questions about cars, crime and law enforcement. Can you tell me about Carroll v. United States and what it meant for the rights of drivers?

Before the Carroll case [in 1925], law enforcement needed a warrant to make a search. That became impracticable with cars because they could be driven off in a moment and there was no time to get a warrant. So the Supreme Court, in the Carroll case, created a warrant exception to say that an officer doesn’t need a warrant to stop and search a car if they have reason to believe that there’s evidence of a crime or contraband inside the car.

For a misdemeanor, police officers also needed to see that misdemeanor taking place with their own eyes, which basically means they knew that a crime was being committed. By saying you don’t need a warrant, even for misdemeanors, that basically watered down the standard for when police officers could act as well. So essentially, the Carroll doctrine really transformed and expanded law enforcement powers. It was the first time that the court constitutionally recognized and authorized police discretion. It gave officers the discretion to act to search if they had reason to believe that there was evidence of a crime, rather than actually knowing it.

This is where the traffic part of the story becomes important. Because pretty quickly, there are so many traffic laws and so many traffic violations and now officers actually don’t even need probable cause under Carroll to stop a car, they just need to see a traffic violation.

In Mr. Nichols’s case, they didn’t have any reason to suspect him of anything, right? They had a hunch. Maybe they just wanted to pick on somebody. They didn’t even have probable cause to believe that he had guns or drugs in his car. So what did they do? [They pulled him over on a] traffic violation.

Practically speaking, what does this mean for Americans’ Fourth Amendment rights, which are meant to protect us and our property from “unreasonable searches and seizures”?

I think the best way to answer that is to actually quote from law enforcement themselves. There’s a text that I analyze in my book called Tactics for Criminal Patrol, and that book says the Fourth Amendment is your tool. Law enforcement are trained to view the Fourth Amendment as not a protector of our privacy rights but as a tool to do what they want.

In an automotive society, it is very watered down. It doesn’t do much.

Wait, how could it be their tool? How could it be interpreted as anything other than a protection of the rights of an individual against unreasonable searches?

Because they have strategies for complying with the Fourth Amendment, and the strategies that they use under the Fourth Amendment are authorized. The Fourth Amendment gives them a veneer of legitimacy.

A familiar pattern in these cases is they’ll stop someone for a minor traffic violation. They can’t search the car at that point still, because Carroll says you need probable cause to search the car. So the traffic stop becomes a moment where they have to gather facts that allows them ultimately to get to search the car. So there’s a doctrine of consent. That is, the police can say, “Can I search the car?” And if you say “sure,” you’ve given your consent, the Fourth Amendment doesn’t apply.

There’s all these power dynamics involved in getting consent. A lot of people don’t know that you can say no. And the Court held that under the Fourth Amendment, the police don’t have to tell drivers that they have the right to say no. So under Fourth Amendment law, as the Court has explained it, the police can pull somebody over for a minor violation, ask to search the car without telling them they have the right to say no, and then [if they say yes] search the car, and it’s given legitimacy to the entire interaction.

I think people are also confused about what the cops can or can’t do when they pull you over. You mentioned the Jay-Z song “99 Problems” in your book. He talks about how his “glove compartment is locked, so is the trunk in the back and I know my rights so you gon’ need a warrant for that.” Is that right? What should people know about what cops can and cannot do when they pull you over?

That “you need a warrant for that” line is wrong, because of the Carroll case. What I always tell my students is when you’re pulled over, and it’s not just about a traffic citation, ask the officer, “Am I free to leave?” because unless law enforcement has at least facts supporting reasonable suspicion that a crime is afoot, they have to let you leave.

If they want to keep you longer than that, always ask, “Am I free to leave?” And if they say no, then ask “Why not?” That’s not a huge help, because there’s so many things that law enforcement can say to amount to reasonable suspicion.

One way that police officers have often justified stopping someone for an investigatory stop is to tell the court, “There was an odor of marijuana.” It’s a really easy thing to tell a judge to justify the stop because it’s really hard to disprove it, and judges are very reluctant to disbelieve. There’s a presumption that officers are telling the truth.

One of the things that really struck me reading your book is how many of these questions about traffic, and what makes humans drive the way they do, and about how we keep people safe, were the same exact debates people were having at the advent of the automotive age. We know discriminatory policing is a huge problem. We also know road safety is a huge problem. And what we have now is very selective policing that is extraordinarily dangerous to people. So how do we balance those concerns? What does a system that is safe and equitable, and not sort of randomly deadly for Black drivers, look like?

I have two responses to that. I think a good place to start with any policy is to ask the most vulnerable people, and in this case, it’s communities of color. Because the pedestrian accident statistics, from what I hear from the people in this field, it really affects minority communities more, and there’s more deaths and accidents in minority communities. They are experiencing the worst of both safety issues and the policing issues. What do they think is the best way to strike that balance between safety and curbing police brutality? I think that’s a good place to start.

The other answer to your question is, are there ways to enforce traffic laws or even encourage safe driving without using police enforcement? It might not be possible to completely 100 percent eradicate human enforcement, but how close can we get there? I’ve advocated for traffic cameras, and the separation of police units that investigate crime from those enforcing traffic. Those are the two main policies that I’ve argued for.

Technology isn’t 100 percent discrimination-proof, and we have to also be mindful of things like equitable placement of the traffic cameras, we also have to pay attention to things like what are the fees. We have to pay attention to those policies. And we could consider a whole host of other things, but I think that’s a good start. I really like the idea of separating criminal law enforcement from traffic enforcement because traffic enforcement should be something that no one should be scared of.

Right now, the situation we have is that people of color, drivers of color, are definitely scared of being pulled over. It’s just harrowing. And you also have police officers saying that traffic stops are the most dangerous part of their jobs, and they’re definitely scared of it. We’re doing something very wrong when everybody’s scared of traffic stops. They should be kind of routine encounters: If you speed, you get a ticket, and that’s it. Just to tone down the anxiety of those encounters, I think, would help a lot. And the way to do that is when people know that when they’re pulled over, it’s not a criminal investigation. It’s just a traffic citation. I think that will go a long way to help.