I believe the answer to this question would be either a metaphor, idiom, or alliteration. But this would most likely be idiom because people can interpret different meanings of this phrase negative or positive and can take it personally or extensively.
I think the time of "the road not taken" and "song of the open road" pretty much described in the title themselves. Both tell the story of the importance of personal value and the freedom of making decision.
Based on that, the answers would be : - Finding one's direction in life - Embracing the unknown
