I based that statement on two sources, neither of which I can find now. The first was something I read about a year ago on the impact of smartphones on police violence. It had data showing a decline in police violence starting around 2011. The second was a comment in The Economist magazine about two weeks ago, in which an academic stated that her research showed a 20% drop in police killings of blacks between 2015 and 2019. I searched through The Economist website but could not locate the quote. Sorry.