There are racial disparities in police stops—Blacks are stopped twice as often as Whites—but they aren’t related to traffic safety offenses, in which cops exercise a little less discretion and violations are equal within groups. Where we see a difference—even after we adjust for driving time (on average, Blacks drive more and longer than Whites)—is in investigatory stops. In these, drivers are stopped for exceedingly minor violations—driving too slowly, malfunctioning lights, failure to signal—which are used as pretext for investigations of the driver and the vehicle. Sanctioned by courts and institutionalized in most police departments, investigatory stops are aimed at “suspicious” drivers and meant to stop crime, not traffic offenses. And as the authors note, “virtually all of the wide racial disparity in the likelihood of being stopped is concentrated in one category of stops: discretionary stops for minor violations of the law.”
The difference between the two kinds of stops is dramatic. Where traffic safety stops are mostly painless (other than tickets), investigatory stops involve searches, impromptu interrogations, and occasionally handcuffs and weapons.