It turns out that when the dad fell asleep at the wheel in the beginning of the movie, he ended up crashing the car into the mysterious lady in white that they kept seeing throughout the movie. This car crash between them killed the entire family instantly (Brad Miller, Richard Harrington, Laura Harrington, Frank Harrington, as well as the lady in white and her little baby daughter). The only one to survive is Marion Harrington, and she is pregnant.
The whole entire drive down that quiet road after the crash was all a nightmare that Marion had and the rest of her family was already dead. But she doesn't know this, it seems like they are still alive and her dream seems so real to her. When, in her dream, it was only Marion left alive, she is confronted with the black car she kept seeing, which is a hearse. The lady in white says to Marion, "He's not here for you" as the lady in white gets into the car. Marion then wakes up in the hospital.
The doctor taking care of her was Dr. Helen Marcott. Marion worked that name into her dream from hearing the name, hence the Marcott sign that the family saw and never made it to. So in all, it was basically "Death", if you will, coming to claim the family's souls one-by-one. The reason that they seemed to be on the never-ending road is because you can say that it was almost a state of purgatory/limbo that the family would be in until every soul was claimed. When the lady in White is shown at the end, and tells Marion he's not here for her, it's her saying that she will live. Then Dr. Marcott meets up with that guy in the black car (who discovered the accident on the road) and told him she liked his car. Hers wouldn't start, so she got a ride from him - becoming his next victim.