Stafford gives us a pretty clear title here, and the fact that this title is repeated as the first half of the poem's first line tells us that he wants us to pay particular attention.
The title sets the scene both literally and thematically. The speaker is driving at night—traveling through the darkness. The title also gives us the sense of "darkness" as in the unknown and suggests our travels through the mysteries of life, death, and the great beyond.
Traveling in the dark also sounds kind of dangerous and difficult—hard to see where you're going in the dark, right? It's also hard to choose the right path when you can't see your hand in front of your face. Stafford wanted all these things to pop into our heads (at some level at least) when we read the title.
When the title is repeated as part of the first line, it gets more grounded in the literal narrative of the poem. The same words as part of the first line seem more concrete. In that way, the title and first line together give us a kind of mystery-to-clarity movement. The title is more abstract, without all the deets of the story to come, but when we see it in the first line we sigh a bit in relief (don't you?). We're not totally alone here, wandering around in the dark and banging our shins on unseen furniture. The story to follow reminds us that we have the speaker, and Stafford through him, to help guide us along.