Actually, though the same music was originated in Badlands, which was also about a young couple on the run, the description sounds more like True Romance, which also used a similar theme extensively. -
The version in True Romance is not the original but Hans Zimmers reworking of it. However, that same theme has been used and plagiarised endlessly on various tv adverts, and used in a few other movies.
This is the original from Badlands...incidentally True Romance was a kind of homage to this movie anyway -
The original piece is called Gassenhaur by Carl Orrf.