Obviously, from the title, the poet is referring to the road not taken. The speaker continues to talk about the road not taken. He is telling this with a sigh. He is wondering about the road he did not take. Would that have made a difference in his life as well? He took the road less traveled by and claims that it has made all the difference, but the speaker is still wondering about the road not taken. In the beginning of the poem, the speaker admits that he is sorry he could not have traveled both roads:
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
Clearly, the speaker is torn. He desires to take both roads. Nevertheless, he decides to take the road less traveled. And yes he is telling this with a sigh. He is saddened by the fact that he could not take both roads, but considering how way leads on to way, he doubts he should ever return to take the "other" road. So the speaker can only wonder about the "other" road. Likewise, the reader wonders what the "other" road would have had in store for the traveler.
In the end, the speaker decides that he took the road less traveled by and that road has made all the difference. There is a sense of satisfaction in the speaker's voice. He made the right choice, but still has a sense of wonderment as to what the "other" road would have held for the speaker.
No doubt, the poet intended for his reader to be just as curious as is the speaker about the "other" road:
Like many of his poems, it seems simple, but it is not exactly straightforward, and even perceptive readers have disagreed considerably over its best interpretation. It looks like a personal poem about a decision of vast importance, but there is evidence to the contrary both inside and outside the poem. Frost has created a richly mysterious reading experience out of a marvelous economy of means.