This title practices the KISS rule: Keep It Simple Stupid. No fancy-pants phrases or obscure allusions to W.B. Yeats here. McCarthy simply names the book after the dominant setting: the road. (Compare McCarthy's title with other recent ones like Dave Egger's memoir A Heartbreaking Work of Staggering Genius or Jonathan Safran Foer's novel Extremely Loud and Incredibly Close. McCarthy's title seems positively quiet next to those two.)
It's useful to think of McCarthy's title as a reflection of his writing style in The Road (see "Writing Style" for more). Like Ernest Hemingway, he maintains a steady and simple style. We think the really have something in common. Specifically, the reticence of McCarthy's title strikes us as very Hemingway-esque, kind of like the classic Old Man and the Sea.
The title also highlights the book's theme of transience. Notice that the characters in The Road never stay longer than a week in any one house or shelter before getting back on the road. All the houses have been abandoned; domestic life has pretty much been obliterated.
It's also worth thinking about destinations in the novel. If the title points us to the American highway, we should also ask just where the characters end up. Answer: it's possible they don't end up anywhere. Their goal of reaching the coast, some might say, turns out to be an empty one.