If I had time to draw and shoot, I had time to get the (insert 4 letter word here) out of the way. The human body is MUCH more maneuverable that a car. So if they're more than say, 20-30 feet away, getting out of the way should be simple. If they're closer... good luck. You're getting ran over... whether you draw you pistol or not.
Assuming you already had your gun drawn at a motorist (for a legitimate reason of course), and THEN they decided to come towards you, I believe shooting would be justified. However, I'd still choose to just get out of the way, depending on the cars speed and distance..... If the car is close enough that you can get out of the way, it will still have momentum, dead driver or not, and hit you. You'll need to move out of the way.... something you could have done in the first place.
Getting out of the way, if possible just makes sense to me.