If I had entrepeneurial spirit I would start a company called CopCam. A cheap but good camera, very recognizable as a camera with a red light on the front and big letters COPCAM. You’d attach it to the dashboard and it would come with a short overview of legislation per state (or european country depending on the market) of your rights as a citizen to film a traffic stop. I would hope cops would recognize it as a sogn they stopped someone who is well aware of their rights and os filming the whole thing, and this way they would behave.