I don't see how either of you can say the car was or wasn't moving given the motion of the camera. If you've done a frame-by-frame analysis (or seen one) by all means share it. Clearly, the guy had put the car into drive and had his foot on the accelerator, as you hear the engine and the car drives down the road a ways before hitting a light pole.
The issue then would be imminence, I guess, if the cop's arm is stuck (or even if he's holding on and refuses to let go, which I think he has a right to do), how long does he have to wait before it becomes reasonable to believe he's facing imminent death or great bodily injury? Should the cop have to wait until the car is moving more than five miles per hour? Ten? Perhaps he has to give the guy five seconds to stop the vehicle?
Is the analysis different if it's a car instead of any other deadly weapon? If he'd pulled a gun does the cop have to wait until it's pointed at him, until the guy fires a shot at him, until he's actually shot?
The cop's arm never should have been in the car in the first place. That's a big no-no. If the cop's life was in danger, IF, it's because he violated protocol for a traffic stop and put his own life in danger. Fuck him.
How about 1 mph?