You must reasonably have a chance of survival, if you start driving and the person would open fire, he would realistically hit you through the window. If you are already in motion you have a higher chance of avoiding said bullets. Though motion does need to be of significant speed. Going 5mph with popped tires is not valid. But already driving 15-30 mph and still accelerating is fine. If you were to hit a wall/pole or other object, causing u to stop, then you will still need to comply.
There is also a difference between a mini cooper and a mazda furai, if you are driving 10mph with a mini, stop.
If you are driving 10-15 mph with a furai, then yes.
Its mostly up to discretion of the staff member looking into a situation, but the above is a general, unspoken guideline most staff if not all staff will follow and look into.