From the article
In 2014, three sociologists at the University of Kansas surveyed more than 2,300 drivers in and around Kansas City. They discovered that while stops over traffic safety violations showed little racial disparity, when it came to stops related to minor violations, like expired license plate stickers, black drivers were pulled over twice as often.
These “investigatory stops” provided what Slate’s Jamelle Bouie describes as a “pretext for something more sinister”:
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.