All your examples have clear line, but, IMO, the placement of the line is too centric.
and, except the dune trail, the line cuts against the horizon.
I feel quite awkward composition.
We call this leading lines. Not all of them need to be straight or centric.
http://www.photips.com/photography-tips/leading-lines.html
More examples:
http://www.brighthub.com/multimedia/photography/articles/959.aspx
Just search for "leading lines in photography" at Google, you will find a lot of examples.
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