"On one occasion he [RF] told of receiving a letter from a grammar-school girl who asked a good question of him: 'Why the sigh?' That letter and that question, he said, had prompted an answer.
Amherst Mass April 1925

"Dear Miss Yates:

No wonder you were a little puzzled over the end of my Road Not Taken. It was my rather private jest at the expense of those who might think I would yet live to be sorry for the way I had taken in life. I suppose I was gently teasing them. I'm not really a very regretful person, but for your solicitousness on my behalf I'm
your friend always
Robert Frost"

[Finger, L. L.: "Frost's 'The Road Not Taken': a 1925 Letter come to Light", American Literature v.50]

"(a) One stanza of 'The Road Not Taken' was written while I was sitting on a sofa in the middle of England: Was found three or four years later, and I couldn't bear not to finish it. I wasn't thinking about myself there, but about a friend who had gone off to war, a person who, whichever road he went, would be sorry he didn't go the other. He was hard on himself that way. (RF, Bread Loaf Writers' Conference, 23 Aug. 1953; tape recording)."

"(c) Frost said that he wrote the poem, 'The Road Not Taken' for his friend [Edward Thomas] and sent it to him in France, getting the reply, 'What are you trying to do with me?' (L. Mertins: Robert Frost)"

[Thompson, Lawrance: Robert Frost: The Years of Triumph, Notes.]

"Although it had been these walks with Thomas which inspired 'The Road Not Taken,' the image of the two roads occupied Frost's mind earlier when he wrote to Susan Hayes Ward on February 10, 1912:
'Two lonely cross-roads that themselves cross each other I have walked several times this winter without meeting or overtaking so much as a single person on foot or on runners. The practically unbroken condition of both for several days after a snow or a blow proves that neither is much travelled.'
That the above image was a catalyst for this poem is partially confirmed by his once dating the poem '1912? 1914'."

[Cramer, Jeffery: Robert Frost Among His Poems]