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The big question about why police pull over so many black drivers

President Obama made remarks to police officers in Chicago, saying that police officers are "scapegoated" for failures of the criminal justice system. (Video:
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Last October, President Obama told a gathering of police chiefs in Chicago about one way he had experienced racial bias before coming to the White House — while driving on the road.

"[M]ost of the time I got a ticket, I deserved it. I knew why I was pulled over. But there were times where I didn't," he said. "[T]here are a lot of African-Americans — not just me — who have that same kind of story of being pulled over. ...[T]he data shows that this is not an aberration. It doesn't mean each case is a problem. It means that when you aggregate all the cases and you look at it, you've got to say that there’s some racial bias in the system."

The fact that blacks get pulled over more than whites while on the road has long been a fact of American life. And on Wednesday evening, that reality became deadly when a Minnesota police officer shot and killed a black driver following a routine stop for a broken tail light, just one day after another fatal police shooting in Louisiana. (The violence continued in Dallas Thursday night, when snipers killed five police officers and injured more.)

While Obama is correct in asserting that blacks are disproportionately pulled over, it's actually rather hard to assess how much of the disparity is driven by racial discrimination by police officers. Data on driver race in police records is sparse, and it's hard to capture what's happening on a national scale. But what research has been done reveals a few key trends:

1)survey released by the Bureau of Justice Statistics in 2014 found that nationwide, 13 percent of black drivers were stopped at least once by police during 2011, the last available data. For white drivers, that figure was 10 percent. Since this only captures whether drivers were stopped once, the disparity could be much bigger if black drivers were stopped more frequently than white drivers.

Though the BJS has not released more recent data, some individual states have these statistics, giving us reason to believe this pattern hasn’t gone away.

A study of Connecticut traffic stops in 2014 and 2015 — among the states with the most recent data — found that 14 percent of traffic stops targeted black drivers, despite them making up only 9 percent of the state’s population. Whites, on the other hand, made up 70 percent of the traffic stops, but about 80 percent of the population. They also found the racial disparity was larger during the day than at night, when the officer can easily observe their race before turning on the sirens. The analogous statistic was not available for white drivers.

2) The racial disparity isn’t just limited to stops. Other police-driver contact — searches, tickets, arrests and license suspensions — show similar racial skews. A 2015 New York Times analysis found “wide racial differences in measure after measure of police conduct” in Ferguson, Mo., as well as in Connecticut, Illinois, North Carolina and Rhode Island, the states with the most comprehensive traffic stop data.

The Times analysis found that among the largest police departments in each of those four states, black drivers were between 1.5 and 5.2 times more likely to have their cars searched than white drivers.  These searches occur with the consent of the driver, so the officer doesn't need to meet any legal standard, like probable cause, to initiate one. Of course, he would need cause to pull over the driver in the first place, though minor violations like going a few miles over the speed limit or shifting lanes without a turn signal can offer that.

3) These disparities extend to other types of traffic stops. As far as tickets, a study of traffic stops in Bloomfield, N.J., released in April uncovered a high level of discrimination in ticketing. The majority-white town issued more than 80 percent of its tickets to black and Latino drivers from September 2014 to August 2015.

There are limits to what one can infer from one city, but same pattern was found across Connecticut. A 2015 analysis by the Hartford Courant found that black and Hispanic drivers who were stopped were 11 to 41 percent more likely to be ticketed than white offenders, depending on the offense committed.

And suspended licenses tell the same story. A report released earlier this year found that in California, black and Latino drivers are more likely than white drivers to be arrested for driving on a license suspended because of unpaid tickets, and minority neighborhoods have higher rates of license suspension than white ones.

4) While the data show conclusively that blacks have a disproportionate number of encounters with police at traffic stops, it hasn't shed much light on what portion of the disparity in all these metrics is driven by racial discrimination, and what portion is attributable to non-racial factors.

According to the National Institute of Justice, the research arm of the Department of Justice, other factors could include a difference in driving patterns — that is, if black drivers tend to just drive more miles than white drivers in certain jurisdictions, they could be stopped more often in those jurisdictions but have the same number of stops per mile. That would indicate against racial discrimination.

Further, black drivers could disproportionately drive in areas with a larger police presence, so they’re more likely to be pulled over than white drivers in less policed areas even without racial profiling by the police. Of course, if this is the case, it’s a bit of a chicken-and-egg problem. Are blacks pulled over disproportionately because they drive through highly policed areas (not discrimination), or are police highly packed into those areas because there are more black drivers (discrimination)?

The NIJ also cites a possible difference in committing stoppable offenses. Though many studies find no difference between whites and blacks in driving habits, there is some evidence that seat-belt use is lower among black drivers and passengers, which could prompt disproportionate stops and tickets.

5) The situation gets even more complicated with respect to vehicle searches. In this case, it's less likely that these other factors are at work. It's more likely either statistical discrimination (targeting black people because they're more likely to be carrying contraband) and outright prejudice (targeting black people because of animus toward them). The first would indicate that black people are searched more because they’re more likely to carry contraband, while the latter would say that black people are searched more often because the police have an antagonism toward them.

It’s an important difference — the former is a result of the police maximizing the number of offenders who are caught, whereas the latter is a result of the police seeking to punish black drivers. The former — what people usually mean when they say "racial profiling" — is ethically controversial for literally targeting people of certain skin colors, but sometimes defended as merely an efficient use of resources. The latter, not so much.

In simple terms, the impact of these two types of discrimination can be compared by looking at the rate people of each race are caught with contraband after they’ve been searched. If black drivers are consistently less likely to be caught with contraband than whites, that means the police are spending their time searching less-suspicious blacks rather than more-suspicious whites, indicating prejudice.

A 2006 study of police searches in Florida found no evidence of prejudice, but evidence of statistical discrimination. More recent figures from 2014 and 2015 published by the New York Times show blacks who are searched are around 20 percent less likely to be carrying contraband than whites who are searched. While that certainly raises the odds that prejudice is playing a role, it can't be said for certain without a more careful study.

To some, however, the distinction between statistical discrimination and outright prejudice isn't so great. “Racial profiling sends the dehumanizing message to our citizens that they are judged by the color of their skin,” reads a fact sheet from the Department of Justice. It cited a “moral obligation” to prohibit the practice. In other words, the department finds both statistical discrimination and prejudice to be unethical.

But others argue that racial discrimination in law enforcement is a cost-effective way to fight crime, a way to apprehend the most guilty people with the fewest officer hours. New York City, under Mayor Michael Bloomberg, fought to defend its “stop, question, and frisk” policy in court. Officers disproportionately stopped blacks by a large margin under the policy; that is, the officers statistically discriminated against them. The city's defense in court focused on the policy's impact — how many weapons were confiscated, how much crime had fallen — and less so on the racial disparity. The policy was struck down as an illegal search in federal court in 2013. The city filed an appeal at first, but Mayor Bill DeBlasio withdrew it after he assumed office, ending the practice.

Ethics aside, this is where the research leaves us: Black drivers certainly get more face-time with traffic cops. But to what extent that reflects discrimination, and whether that discrimination is based in racial prejudice, is more of an open question.

The officer who fatally shot Philando Castile, 32, has been charged with second-degree manslaughter. Here's what you need to know about the traffic stop gone wrong, which ended up live-streamed on Facebook. (Editor's note: This video has been updated.) (Video: Monica Akhtar, Jenny Starrs, Gillian Brockell/The Washington Post)