This is a fantastic poem and a great question to think about. I would say that the title is working on several levels of meaning. The obvious literal meaning refers to the persona's journey by car along a dark road, but "the dark" also refers to the difficult emotional situation which the persona encounters. The doe is dead, but her unborn fawn is still alive inside. The persona decides to push the doe and fawn over the cliff so that there is no longer an obstruction on the road, but the persona clearly struggles with the knowledge that they are finally killing the unborn fawn. You can see this struggle in the tough, terse diction Stafford uses such as "Beside that mountain road I hesitated" and his focus on the animalistic car with "its lowered parking lights." This is an example of a transferred epithet, as the poet seems to be describing the persona's sensation of being a ruthless killer rather than the car itself.
The title "Travelling through the dark" also seems to be a metaphor for life itself, so that the story of the poem, the persona's struggle to deal with the deer humanely and responsibly, becomes representative of the struggle to survive in life. We often have to face tragic situations which require our courage and strength. This poem seems to both celebrate human resilience and explore the sadness of coping with the realities of death. There are definitely more things to say about the title. You'll be able now to have another look and see what else you might say.