A dead metaphor is one that has lost its original meaning due to overuse and the passage of time. In Thomas Lux's poem, the wife is driving on a road "that cuts straight north" (line 6). This phrases provides a variety of angles for analysis. First, if we are to classify this phrase as metaphorical, I would haven to say that it is a simply "normal" metaphor. It is not a phrase that has been overused because we just don' t see very many examples of it. It also carries no meaning based upon situations from previous years.
However, in this poem, the phrase takes on additional metaphorical meaning. The use of the word "cut" makes the reader get a sense of a knife or even violence of some kind. The paper companies have clearly cut away many trees and other growth to make the large roads.
While she is on the road "not fat enough for the paper companies," (line 8) she is in an area the moose considers home. Cutting his territory in two, as her road is doing, is a metaphorical act of violence not only against the innocent moose, but against all of the indigenous animals as well.
Thus this metaphor ties into a great theme of human beings destroying nature and the negative consequences therein.