What is the significance of the colon in the last stanza of "The Road Not Taken?"
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I,
I took the one less traveled by,
And that has made all the difference.
The colon in the last stanza of "The Road Not Taken" causes the reader and speaker of the poem to pause. It also separates the poet's present monologue with what he will say "ages and ages hence." In other words, lines 1-15 are all spoken as one narrative. Lines 16, 17 and 18 lead into the lines after the colon. Lines 19 and 20, the concluding two lines, are what the speaker/poet assumes he will be saying/thinking many years later when he reflects on the road he took and imagines what his life would have been had he taken the other road.
The em dash, following "and I" in line 18 is an equally significant pause. It may indicate a dramatic or emphatic or reflective pause where the speaker considers the road he did not take; it may be sad or triumphant or reflective, each necessitating a pause. Or, this could be a humorous trick with Frost mocking the speaker's inflated sense of himself and his decision which one could certainly find overly dramatic since both roads looked "really about the same." Any decision "makes all the difference" but who's to say which road is less travelled (better or worse) if both roads are equally "worn." The colon indicates a proposed skipping ahead to the future when the speaker will again pause to consider the actual implications and supposed missed opportunities of his decision.