A close look at the poem reveals that Frost's walker encounters two nearlv identical paths: so he insists, repeatedly. The walker looks down one, first, then the other, "as just as fair." Indeed, "the passing there / Had worn them reallv about the same." As if the reader hasn't gotten the message, Frost says for a third time. "And both that morning equally lay/ In leaves no step had trodden black." What, then, can we make of the final stanza? My guess is that Frost, the wily ironist, is saying something like this: "When I am old, like all old men, I shall make a myth of my life. I shall pretend, as we all do, that I took the less traveled road. But I shall be lying." Frost signals the mockingly self-inflated tone of the last stanza by repeating the word "I," which rhymes - several times - with the inflated word "sigh." Frost wants the reader to know that what he will be saying, that he took the road less traveled, is a fraudulent position, hence the sigh.
From "Frost" in Columbia Literary History of the United States. Ed. Emory Elliott. Copyright © 1988 by the Columbia University Press.