New movie 'The Hitcher'--SPOILER ALERT!!?

I just saw this movie last night, and I have a question, because I missed part of it (had to get up and go to the bathroom!)

After Jim and Grace kick the bad guy out of the car the first time, and they pull over to sleep, Jim wakes up from having a bad dream. He then goes out of the car and talks to Grace.

This is the point where I left.

When I came back, I saw that Jim and Grace were back in the car, and the car had been run off the road by a station wagon and it flips over and wreaks alongside the road.

Then later, they come upon the car, and there are two people in the car who had been stabbed.

My question is, what did I miss? Who are these people and were there any other people in the car (I saw a stuffed animal and a child's book, so I'm wondering if there were children in the car)? Why did the people in the car run Jim and Grace off the road?

WHAT DID I MISS???

Thank you in advance for your answers!!!