BLACK MOTORISTS have long complained that American police officers tend to target them for traffic stops. A study by the Justice Department, published in 2013, found that black drivers were 31% more likely to be pulled over than white ones; other interactions with law enforcement are characterised by similar racial imbalances.

Such figures do not by themselves prove that bias is to blame. It is hard to distinguish how much of this imbalance can be attributed to race-specific differences in driving behaviours—such as adherence to traffic laws or the amount of time spent on roads in areas where police are present—and how much is driven by racial profiling. But new research suggests that discrimination is at least part of the problem.

A study released this week by researchers from Stanford University and New York University finds “widespread discrimination” in police officers’ decisions to stop and search drivers. The authors used data from nearly 100m traffic stops carried out by state and municipal police departments across America between 2011 and 2017. They find that blacks are about 38% more likely to be pulled over by state patrol officers than white drivers. For local officers, the figure is 35%.

At least some of this gap can be attributed to racial bias. Using a “veil of darkness” test, the authors show that black drivers account for a significantly higher share of traffic stops during the day, when it is easier for an officer to determine a driver’s race, than they do at night, when a driver’s skin colour is harder to ascertain (see chart). The variation in sunset times during the year, three of which are shown in the panels above, allow the researchers to control for other factors such as when the most police officers are on patrol.

State and local lawmakers can help combat such discrimination by gathering and analysing information on it. In 1999, North Carolina became the first state in America to require police agencies to collect and report data on traffic stops. Other states, including Connecticut and California, have followed, but some do not yet do so. The authors of the study argue that such data-collection efforts will help police officers to do their jobs better. “Collecting, releasing, and analysing police data are essential steps for increasing the effectiveness and equity of law-enforcement practices,” the authors write, “and for improving relations with the public.”