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5-Year Study Shows Police Stop Black Drivers Less Often at Night When ‘Veil of Darkness’ Obscures Race

In the largest-ever study of racial profiling by police during traffic stops, Stanford University has shown that Black people are much less likely to be stopped after sunset when “a veil of darkness” masks their race. The five-year study analyzed 95 million traffic-stop records that had been filed by officers from 21 state patrol agencies and 35 municipal police forces from 2011 to 2018.

The study was a collaboration between Stanford’s Cheryl Phillips (a journalism lecturer whose students obtained the raw data through public records requests), Sharad Goel (a professor of management science and engineering whose computer science team organized and analyzed the data), and Ravi Shroff (a professor of applied statistics at New York University who worked with Goel).

The team spent years culling through the data, eliminating records that were incomplete or from the wrong time periods (focusing on 7:00 p.m. local time when the sky is lighter or darker depending on daylight savings time), to create the 95 million-record database. The dataset provided a statistically valid sample with two important variables: (1) the race of the driver being stopped and (2) the darkness of the sky. The analysis left no doubt that the darker it got, the less likely a Black driver would be stopped. The reverse was true when the sky was lighter. The study confirmed the results of a much smaller study of 8,000 drivers in Oakland, California, over a six-month period in 2006. But the Oakland study was dismissed as “inconclusive” because the study was considered too small to prove a connection between the darkness of the sky and the race of the driver.

The collaborative study also found that when drivers were pulled over, officers searched the vehicles of Blacks and Hispanics more often than Whites.

Researchers included a subset of data from Colorado and Washington after legalization of marijuana in those states and found that, although vehicle searches declined after the legalization, police continued to search Blacks and Hispanics more frequently than Whites. “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 wrote in Nature Human Behavior.

Using the Stanford Open Policing Project, the researchers make the data available to reporters and hold workshops to teach them how to use the data to do local stories. Researchers helped reporters from the Seattle-based nonprofit news organization Investigate West use the data for stories that resulted in the Washington State Patrol reviewing its practices and boosting officer training with regard to interactions with Native Americans. The researchers similarly helped reporters from the Los Angeles Times use the data which ultimately resulted in a series of stories that prompted changes in the practices of the Los Angeles Police Department concerning stops and searches of minorities. “All told[,] we’ve trained about 200 journalists, which is one of the unique things about this project,” said Phillips. Goel and Phillips continue collaborating through a project called “Big Local News” to explore how data science can shed light on public issues like civil asset forfeiture. 


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