The chance that police stop a typical American driver in a given year is about 10 percent, according to federal data. Comedian Chris Rock has been stopped three times in less than two months, and he's been posting pictures of each incident to social media.

Stopped by the cops again wish me luck.

A photo posted by Chris Rock (@chrisrock) on

It isn't clear whether Rock was breaking any rules of the road (on one occasion, he wasn't driving), but it is clear that as a black man, he is far more likely to be pulled over by police. The data from the Justice Department show that black drivers are about 23 percent more likely to be pulled over than white drivers. Native Americans are stopped most frequently of all.

It's not as though black drivers don't obey traffic laws, and white drivers do. The federal survey found that relatively speaking, far fewer blacks than whites were pulled over for speeding. Instead, the cops stopped them because of a "vehicle defect," to check a record, or for some other or unspecified reason.

Black drivers were also about three times as likely as white drivers to be searched after they were stopped.

One likely explanation for these trends is that police tend to make two different kinds of stops on the road, as Charles Epp and Steven Maynard-Moody have documented in the Washington Monthly. When cops see a traffic violation, they pull the driver over, write a ticket and send her on her way. When they see something vaguely suspicious, they might stop the driver, make conversation, ask a few questions and then see if they can get permission to search the car. This kind of stop is often called "investigatory," write Epp and Maynard-Moody, professors at the University of Kansas.

They conducted a survey of 2,329 drivers in Kansas City, and found that police were equally likely to pull over black and white drivers for traffic violations. Investigatory stops were the reason that blacks were pulled over more frequently. A typical black man under the age of 25 had a 28 percent chance per year of being stopped without having broken the rules of the road, compared to 12.5 percent for a white man of the same age with similar driving habits.

Kansas City is probably typical of the nation as whole, Epp and Maynard-Moody write, but they emphasize that cops themselves aren't necessarily to blame for the disparity. "The underlying reason for this is not racism by individual officers," they write. "Rather, it is police department directives requiring officers to make large numbers of stops just to check people out." Police administrators want to find dangerous people traveling with guns and drugs before they commit crimes, but the only way to do so is to stop lots and lots drivers and search their cars.

As a result, cops pull people over even when they have no real evidence to go on. Black drivers feel like they're being harassed by racist officers. As Wonkblog has reported in the past, however, there are many reasons that police may be more likely to stop and arrest people of color that have nothing to do with racism among cops or criminal activity among minority groups.