I think the movie pretty much gives the solution for the ending.the girl has tried to escape but within the family rules.it is not important if she will escape because i do not think she knows what escape really means.she does not have the tools to make it to the outside world anyway.it is quite impossible to understand how she functions,and which rules are governing her actions.i think that the end just shows that her world is as empty and dark as the back of the car where she is.it is our own need to translate the movie into our world that we think of escape.it is of no importance if she 'makes it'or not.and that is the concept of the movie.but i also think that the director does not show her going out (whether she can or cannot) because that would mean a sense of freedom,and the movie's concept is totally against that kind of notion.
