Try on this theory

How about this:
The boy was in a class with the girl a year before. He had a crush on her but was too afraid to approach her.

The Christmas before, he is traveling on that road, and skids off into a snow bank and dies. Exactly the way we see it happen in the movie but a year before.

The girl is not be aware of his death.

i think he is haunting her. While he is haunting her in class and everywhere else he learns everything about her. That is why he knows she is without a ride, she wears glasses, and what food she likes.

This road trip is his chance to be with her. He is definitely a ghost. How he is able to appear to her and drive is a mystery.

i agree that everyone in the gas station is a ghost. The reason the bathroom is so bad and the door doesn't open is because it has been closed down for some time. She is the only human in the station but for reasons we don't understand she is part of this ghost world.

The boy takes the same route he took when he had his accident. After his accident he learns the history of this portion of road and is going to do the same thing that happened to him. She will freeze to death like he did and they will be in eternity together.

But it doesn't work out right. he is unable to keep her from scared almost to death and she doesn't die when he does.

He changes his mind at some point and leads her to safety.

***Or the same scenario could work with him auditing her class just to be near her and finding out he has a chance to go on this road trip with her.
What luck! So when he stops at the station where all the ghosts are eating, the reason he can talk to the attendant is the attendant is alive. she got stuck in the bathroom because it was old.

He takes the short cut, to have private time with her, and you know the rest.
He decides not to let her die and be part of his eternity, but leads her to safety because he really did care about her.

That will be the last time she does not fly home....