Robbery is a theft committed in the presence of the victim -- a hold up. As the legal lexicographer John Cowell wrote in 1607:
Robberie commeth from the French, and in our common lawe [signifies] a
felonious taking away of another mans goods from his person or presence, against
his will, putting him in feare, and of purpose to steal the same goods.
Highway robbery was, at first, not a metaphor, but a technical offense -- a robbery committed on the King's highway, and it carried a heavier sentence.