Where It All Goes Down
Setting is crucial in this one. All the setting elements (nighttime, the wilderness, the mountain road, the car) are necessary for this poem to work. Just try to imagine the poem without one of these elements and you'll probably agree that some aspect of the poem would be missing or at least dramatically diminished.
For example, let's imagine everything in the poem is the same, except it is daytime. We lose all those associations with darkness we talked about in the "Symbols, Imagery, Wordplay" section. We lose that eerie red glow from the tail-light, and we lose the intense focus of the scene (if it is daytime, we can see the trees and the mountain and the birds and the river—at night, all we can see is what is illuminated by the red tail-light: the dead deer and the speaker). Or what if the road was wide and lined with flower-filled meadows instead of being narrow and on the edge of a canyon? We would lose that sense of teetering precariously on the edge of life's road, overlooking death's dark abyss.
In a sense, the setting is the poem. The loneliness, the sadness, the emotion of the poem comes from the setting and the action, not from the language. Think about it. The most emotional the speaker gets is in the line "I thought hard" (17). Almost everything we feel, almost everything we are forced to consider comes from setting, situation, and the action that is described—not from the speaker's feelings or language.