Businessman Pulled Over By Police While Driving Car

(© Andrey Popov -

STANFORD, Ca. — As Americans react to the tragic and senseless death of George Floyd, local police departments have once again taken centerstage. Is systemic racism something that plagues precincts across the country? Researchers at Stanford University led the largest-ever study specifically focusing on racial profiling during traffic stops. They say that while black drivers are more likely to be pulled over than white drivers during the day, the shade of night reduces the likelihood that they’ll be stopped because “a veil of darkness” masks their faces.

The term “veil of darkness” was first used by a study of traffic stops in Oakland, California in 2006. The study of 8,000 drivers tried to identify a relationship between the race of a driver and the likelihood they would be stopped at any time of day. The findings of that study were inconclusive, so researchers followed up with a much larger study, reviewing 95 million traffic stop records from 2011-2018.

“Our results indicate that police stops and search decisions suffer from persistent racial bias, and point to the value of policy interventions to mitigate these disparities,” the researchers write in their journal article, which is published in Nature Human Behavior.

The authors created their database of traffic stop records filed by officers with 21 state patrol agencies and 35 municipal police forces. The database only includes traffic stops that took place around 7 p.m. because, depending on the time of year and daylight saving time, it can either be quite light or very dark at 7 p.m. This ensured that there were fewer variables that could cloud the findings of their study.


The analyses show that not only are blacks more likely to be stopped during times when it is lighter outside, but once police officers have stopped a car for a traffic violation, they are more likely to search the car if the driver is black or Hispanic than if they were white. In the states of Washington and Colorado where marijuana has been legalized, there was a reduction in the number of car searches during a traffic stop. However, the data shows that minorities are still more likely to have their car searched after being pulled over.

The results of this study have caused researchers to start a program called the “Stanford Open Policing Project” which makes the data used for the study available to reporters. Researchers even host workshops to help reporters understand how to use these “big-data” databases and identify patterns in the data. More than 200 journalists have been trained thus far. These efforts have led to large investigative reporting projects that have inspired police reform in big cities like Seattle and Los Angeles.

Researchers hope to continue using data science to shed light on major public issues. “These projects demonstrate the power of combining data science with journalism to tell important stories,” concludes lead author Sharad Goel, an assistant professor of management science and engineering at Stanford University, in a university release.

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