What type of literary devices does the speaker have [use] in the poem "The Road Not Taken"?
First, concerning Frost's "The Road Not Taken," the speaker doesn't "have" literary devices, he "uses" literary devices.
And the central literary device he uses is extended metaphor. In literature, roads and journeys often symbolize the roads or journeys of life, and the speaker's use here is no exception. The road the speaker chooses to travel is metaphorically compared to the road he takes in life, and the road he chooses not to take is metaphorically compared to the road he does not take in life.
That said, the road the speaker chooses not to take is really the center of the poem itself. The poem is about regrets concerning missed opportunities. More specifically, the poem is about the speaker's obsession with missed opportunities. He is indecisive and regrets not being able to take both roads, even though doing so is impossible.
The speaker will tell the story years later, as a regret, a "sigh," perhaps a chuckle. He doesn't know what difference his choice of road will make, and the roads are pretty much the same, by his own admission:
Though as for that [there being a difference between the two roads], the passing there
Had worn them really about the same,...
But in the poem's present he can see himself, having obsessed over the difference for years, telling listeners that a great difference existed.
Concerning other literary devices, the enotes Study Guide on the poem says the following:
Frost composed this poem in four five-line stanzas with only two end rhymes in each stanza (abaab). The flexible iambic meter has four strong beats to the line. Of the technical achievements in “The Road Not Taken,” one in particular shows Frost's skill at enforcing meaning through form. The poem ends:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.
The indecision of the speaker—his divided state of mind—is heightened by the repetition of “I,” split by the line division and emphasized by the rhyme and pause. It is an effect possible only in a rhymed and metrical poem—and thus a good argument for the continuing viability of traditional forms.