The narrator of "Traveling Through The Dark" at first assumes he knows all there is to know about the situation he has encountered - a dead deer on the side of the road, a potential hazard to other drivers that can be averted by pushing the body "into the canyon" and out of the path of traffic.
However, he learns there is an unexpected complication to his plan. The dead deer was pregnant and "her side was warm; her fawn lay there waiting, alive, still." Now, with this new awareness, the action of pushing the carcass "over the edge into the river" means killing the fawn as well as removing the traffic hazard.
Stafford suggests that human knowledge may be influenced by conditions and situations - the "narrow...mountain road," the "lowered parking lights" illuminating the way ahead while "the glare of the warm exhaust (turned) red." The narrator senses the potential life represented by the fawn, but also considers the risks to other travelers of the danger inherent in the obstacle on the side of the road.
Finally, the narrator takes action, based on his assessment of the risks and benefits, recognizing that he can't know how the future might have been different if he did something else, and doing the best he can - "then pushed her over the edge into the river."