An officer conducts a traffic stop along Interstate 70 in St. Louis in 2017. (Whitney Curtis for The Washington Post)
Sarah A. Seo is an associate professor of law at the University of Iowa College of Law and author of "Policing the Open Road: How Cars Transformed American Freedom."

All drivers commit traffic violations. But most do not know that, according to the Supreme Court, the police can arrest them even for the most minor infractions, like failure to wear a seat belt. The traffic violation that most commonly lands a driver in jail is driving with a suspended license, which usually results from failing to pay a ticket.

Recent reporting on the topic has sparked discussion over how such arrests are disproportionately concentrated in poor communities, essentially punishing people for being too poor to pay. Compounding their financial problems, many of those arrested can ill afford the snowball effect of overdue fines, court appearances and, potentially, jail time. This has become an all-too-familiar pattern in how the criminal justice system exacerbates inequality.

The history of how we got to this point begins in the early 20th century with the opposite situation: the arrests of the automobile’s early adopters, people with social and economic standing. Though not all were wealthy — by the mid-1920s, a majority of American families owned a car — they were largely white and deemed respectable.

The traffic ticket system actually developed to replace arrests as part of an effort to foster better relationships between these citizens and the police. But it has now transformed into a policy that undermines equality and further erodes poor and minority Americans’ trust in the police.

When mass-produced cars rolled off assembly lines, they created chaos on streets originally intended for pedestrians and horse-drawn carriages. According to the National Safety Council, between 1913 and 1932, deaths from car accidents increased 500 percent. As late as 1935, officials identified reckless driving as “Public Enemy Number One.”

To impose order and to ensure public safety, local officials passed long lists of traffic rules and regulations. In addition to speed limits and license requirements, new laws mandated safety equipment. They also established uniform driving norms by determining, for example, who among cars, horses, carriages and pedestrians had the right of way.

Suddenly, even average citizens became regular misdemeanor offenders. At first, traffic violations were enforced like any other crime. Violators were arrested, taken into custody and brought before a magistrate. But it was impractical for traffic cops to leave their posts to bring every offender to court. Also, full-blown trial procedures for even the most minor traffic offenses swamped the courts.

In some jurisdictions, officers began to hand out summonses, which were notices to appear in court at a later time. By the early 1920s, cities started experimenting with citations or tickets, which became commonplace in the late 1930s. A ticket differed from a summons in that, rather than having to appear in court and go through the criminal process, a person could simply pay a fine at the police station.

The streamlined procedures for traffic offenses, however, created a basic rule-of-law problem. In a democracy, the police were not supposed to exercise judicial powers. In the United States, guilt was supposed to be determined after a fair trial overseen by a neutral magistrate. The ticketing system essentially eliminated the judicial portion of criminal adjudication.

To be sure, defendants could have their cases transferred to court so they could stand trial. But most people preferred to pay their fines at the police station without the hassle of appearing in court. The upshot was that cops got to determine guilt on the road.

Yet ticketing grew in popularity by offering a solution to a difficult conundrum: How to police people not used to being policed. Because most of the drivers in this period were considered “respectable” citizens, arresting them could, at best, create a public relations headache. At worst, their disrespect for law enforcement could undermine respect for the law more broadly.

In fact, one police chief reported that his department lost so much of its goodwill because of traffic law enforcement that even pillars of the community became enemies of the police. Juries reportedly acquitted defendants out of sympathy, despite solid evidence of guilt. Maintaining the public’s support while managing traffic soon became a serious police problem.

The option of issuing a ticket instead of making an arrest helped to pacify an angry driver not accustomed to being stopped by police, let alone getting arrested. Police leaders hoped that tickets, by lowering the stakes of being prosecuted, could defuse confrontations between citizens and the police.

With this priority in mind, officer training manuals recommended that the police exercise discretion when directing traffic. It was up to the individual officer to decide whether to ignore the infraction, give a warning, write a citation or make an arrest. Police consultants advised officers to take the most lenient option or to not attempt enforcement of the law at all for fear of alienating the respectable members of society whose support was crucial for local police departments.

The practice of arresting drivers did not disappear, however. Today, the police’s exercise of discretion targets poor and minority citizens for traffic arrests. As many criminal law scholars have pointed out, in practice, such discretionary policing has enabled discriminatory policing.

Of course, traffic violations must be punished. Our safety on the streets depends on it. Authorizing the arrest of drivers who fail to pay their tickets makes sense on one level. After all, without the threat of some consequence, people may be disinclined to pay their tickets, and without a penalty, they may feel little need to abide by the traffic laws.

But American society once figured out how to enforce traffic laws proportionately and with minimal disruption to people’s lives when traditional criminal procedures negatively affected respectable folks. It is once again necessary to figure out a way to enforce our traffic laws fairly and with minimal disruption to all Americans. Justice demands it.