What should I do when I see a cop drinking and driving?

I know this guy that is police officer, I've known him for years and I see him when he is off-duty in the bar almost every weekend.Sometimes, other people and I witness him getting super drunk to where he is acting belligerent, being very loud and obnoxious and obviously way too intoxicated to drive, but he always drives home and I see him drive. One time, he got super drunk and got into a fight with his girlfriend in the bar then when the bar closed he was yelling at his girlfriend, got in his car, and sped off like going way too fast for a 25mph street with many intoxicated people walking home. I want to say something to him but I don't think he will listen, what should I do when I see this happening again?