Halliday & Matthiessen (2014: 730):
The important point to make is that a piece of wording that is metaphorical has, as it were, an additional dimension of meaning: it ‘means’ both metaphorically and congruently. Thus, to go back to alcohol impairment: here impairment is a noun functioning as Thing, and hence takes on the status of an entity participating in some other process, as in:
Because alcohol impairment effects are well established and documented, alcohol impairment can be used as a benchmark for other forms of driving impairment, such as fatigue, or in comparison to the effects of other drugs.
It does not thereby lose its own semantic character as a process, which it has by virtue of the fact that congruently it is realised as a verb; but it acquires an additional semantic feature by becoming a noun. Compare failure in Engines of the 36 class only appeared on this train in times of reduced loading, or engine failure. – where a more congruent version would be whenever an engine failed. Thus, however far one may choose to go in unpacking ideational metaphor, it is important also to analyse each instance as it is. A significant feature of our present-day world is that it consists so largely of metaphorically constructed entities, like access, advances, allocation, impairment and appeal.