Question: Since Marty's actions led to him not existing, shouldn't no Marty mean that there would have been no Marty to get hit by the car in the first place, meaning that Marty would have just reappeared when he ceased to exist?
JC Fernandez
4th Oct 2009
Answer: The simple answer is NO. According to the time travel rules established in the films, alternate realities are created when changes are made to the past. Marty continues to exist as long as there's the possibility that he exists in 1985. Small changes don't affect him. Marty only begins to disappear after the past has been altered so significantly that he would *never* exist in the present. But at the time he gets hit by the car, Marty hadn't impacted the timeline enough to assure his non-existence.
JC Fernandez