Cars transformed America. They also made people more vulnerable to the police.

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...