I'm reading all these responses and agreeing with a few on what my actions would be (I too would try to render assistance - as I'm single with no kids).
My big question, and perhaps this is because I live in the city (Vegas), is when I stop my truck 100yds from the cop and come out with a rifle, what does HE think I'm going to do? Does the cop immediately think I'm another BG? Sure, I'm not pointing my weapon at him (probably pointed up if I'm behind cover), but he has no idea of my intentions.
My two biggest fears when it comes to this scenario (and we've all thought about it after seeing COPS or something, then driving by a cop with someone pulled over on the side of the road), are either 1. scaring the cop into shooting me because he doesn't know my intentions; or 2. possibly shooting the wrong guy in the situation. I guess if the cop is in uniform, that's a fair assumption that he's the good guy, but what about an undercover? Impersonator?
So, this guy is crouched behind the patrol car wearing plain clothes...is he the perp that made it back to the car and the cop has him pinned down or is he a plainclothes officer or an off-duty? There's where it starts getting gray for me.