I'll start by saying I'm pretty sure this isn't black dog...

There is a guy and a girl in a truck and three semis block them in and make them pull over. They take the girlfriend
Later in the movie the guy finds the truckers and the girlfriend was locked in the luggage compartment under the truck.

http://en.wikipedia.org/wiki/Breakdown_(film) ?

That was my thought, but I don't remember the part about the three semis, although I saw this years ago.

Yeah, I don't remember that part either, but the second part fit so I figured I'd give it a shot.

I will check it out and get back to you... though I trust you quinn cause you are normally right