In season 2, episode 1 of the Waking Dead, there is a scene in which dead people are shown in their cars, not having been zombified. However, we know from a later revelation that everyone is infected and will turn in to zombies when they die. Taking that into consideration, it would seem that those who had died in their cars would have turned in their cars and walked away. However, they are simply rotting corpses sitting in their seats without apparent head trauma to indicate prevention of the zombification process.
