There are many metaphors in "The Highwayman" poem, including "torrent of darkness," "ghostly galleon," and "ribbon of moonlight," and there are many similes, including "like mouldy hay" and "as a dog." The literary device of alliteration is also abundant throughout the poem.
In the beginning of the poem, there is little light in the sky, and the effect is metaphorically described as a "torrent of darkness." That lack of light is partially explained by the metaphor of the moon as "ghostly galleon tossed upon the seas," with the road being described as the only well-lit path. It is metaphorically described as "a ribbon of moonlight, over a purple moor."
Later in the poem, similes are used to describe the manic and haggard appearance of a stableman named Tim. His hair is described with the simile "like mouldy hay," and this complements the description of Tim's eyes as "hollows of madness." The simile "dumb as a dog" is used to describe how Tim is unable to speak as he listens to a robber talk.
Later, the simile "burned like a brand" is used to describe Tim's blushing face as he smelled and felt the hair of his landlord's daughter. A final simile is used near the end of the poem, when Tim is described as being shot "down like a dog on the highway."