The challenge
Cities hoped cameras would curb police use of force and complaints and rebuild public trust.
The case
Washington, DC's Metropolitan Police Department and The Lab @ DC ran one of the largest randomized trials of body-worn cameras. Of 2,224 officers, 1,189 were assigned cameras and 1,035 were not, with deployment from June 2015 to May 2016 and outcomes tracked into December 2016. Officers recorded consistently. Yet the cameras had very small and statistically insignificant effects on use of force, civilian complaints, arrests for disorderly conduct, court outcomes and other measures. The best estimate was actually a slight, non-significant rise in use of force. The study had enough power to detect small effects, so the null result is unlikely to be due to sample size.
Why it didn't work
Cameras alone did not change police behaviour in a department that already had oversight and policies in place. Their value may lie in evidence and transparency rather than in deterring misconduct.
Sources
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