There's no "apocalypse" in this movie. They are in a quarantine zone, and the cars driving around are in the movie because they are supposed to be. I saw no cars that shouldn't be anywhere in the background. Unless you meant in the non-QZ, then that's just poor comprehension skills.
Cars where driving in the quarantine zone like there was nothing check when she is in the van you can clearly see them that was a complete turn off, bad movie really bad movie