My theory/interpretation of this movie

I've watched it a few times so thought I would give this a go. It's similar to some other interpretations of the movie that I've read on here. (Spoilers ahead obviously).

Frank falls asleep at the wheel, and crashes the car into the woman driving with her baby.

They all then enter into a 'limbo/purgatory' dream world. This is sort of like an in-between life on Earth and heaven/hell. Because it is an in-between, it is linked to the physical world so objects and people can actually move between them (such as the dead girl's school-book in the ghost story that Frank tells Marion, and Frank's note).

In the dream world scenario, each member of the family dies in a similar way to the way they died in the car crash. Brad gets mangled, Richard was perhaps engulfed in flames of the explosion, Frank maybe gets something impaled through his leg, and Laura gets a serious head injury (hence why she goes insane). Marion is put into the 'limbo' as well because she is in a coma and is so close to death. She subconsciously hears Dr Marcott trying to nurse her back to health, and this manifests as a destination at the end of this endless road, so she thinks if she can get the family there they will all be saved.

But of course, they have already all died apart from Marion, so 'death' (the man in the old car) claims their souls. At the end, the woman in white tells Marion "He isn't here for you." The woman in white was NOT referring to herself; she was referring to Dr Marcott.

Marion would have died if Dr Marcott hadn't nursed her back to health; hence Dr Marcott cheats death and saves Marion. This drives 'death' (the man with the old car) to cross-over from limbo into the real, physical world, and capture Dr Marcott in his car and steal her soul.

Yeah, it's speculation but that's my interpretation of the movie!