What is the simile in "The Road Not Taken" by Robert Frost?
While both similes and metaphors are types of figurative language, which ultimately entail the comparison of one thing to another thing for the purposes of illumination, the two devices are not identical or interchangeable. A simile compares something to another thing without suggesting that it is that thing: for example, "The road writhed through the valley like a snake." A metaphor, on the other hand, does not overtly compare two things, but describes something as if it is something else. In this poem, the life of the speaker is imagined as a journey or road; the speaker is approaching a fork in the road, which represents a point in life at which the speaker is forced to decide between two options.
In the case of this poem, the speaker takes "the road less travelled by," which here represents a path less usually taken in life. The speaker feels that it has "made all the difference" to his life that he chose a more uncommon route.
I think that you hit on a strong point in the poem. If I may offer one note of clarification, it would be that the poem is rooted in comparative language. I believe that it is a metaphor being used, and not a simile. The metaphor used in the poem is the divergent road, and the idea that the speaker must choose one of the two paths. The metaphor of the fork in the road is a compelling one because it highlights the power and the agony of choice. The speaker is poised between two equally desirable, but ultimately incompatible courses of action and a choice must be made. There is little negotiation in this paradigm, and the metaphor of the fork in the road highlights this. The metaphor compares the two paths to the many different chocies one faces in life. The paths chosen in both the speaker's predicaments, and in our own, "make all the difference" in identity formation. The metaphor hopes to highlight this.