Found on wikipedia; not a direct answer, I'm afraid, but it provides some context:
The title song, "Hard Nose the Highway" is explained by Morrison as: "the theme running through the whole song is 'Seen some hard times' which I have 'Drawn some fine lines' which I definitely have, and 'No time for shoe shines' when you're trying to make a living.
To be hard-nosed is to be no-nonsense, serious, and determined, stubborn, unyielding, and unsentimental. If you have to make a living, you don't have the luxury of stopping: you face the work, the highway, unwavering and without compromise.
I wonder if "hard nose" has a specific meaning regarding cars or driving.
The highway represent the world, possibly, meaning reality and the battle for survival or subsistence. If you see the highway ins the same light as Frost's "promises to keep, and miles to go before I sleep," roads represent obligations.
The song talks about a person who daren't stop moving.