Symbolism, Imagery, Allegory
As far as images of existential abandonment, the flarepistol (or flare gun) takes the cake. What's sadder than shooting a flare into the ocean with absolutely no hope that anyone will see it? To make the situation more somber, McCarthy even brings up the God situation. In The Road it seems like God has left the world to its own devices, which aren't very decent ones. The flarepistol suggests total abandonment: from other kind human beings and from God.
He loaded the flarepistol and as soon as it was dark they walked out down the beach away from the fire and he asked the boy if he wanted to shoot it.
[The Boy:] You shoot it, Papa. You know how to do it.
[The Man:] Okay.
He cocked the gun and aimed it out over the bay and pulled the trigger. The flare arced up into the murk with a long whoosh and broke somewhere out over the water in a clouded light and hung there. The hot tendrils of magnesium drifted slowly down the dark and the pale foreshore tide started in the glare and slowly faded. He looked down at the boy's upturned face.
[The Boy]: They couldnt see it very far, could they Papa?
[The Man:] Who?
[The Boy:] Anybody.
[The Man:] No. Not far.
[The Boy:] If you wanted to show where you were.
[The Man:] You mean like to the good guys?
[The Boy:] Yes. Or anybody that you wanted them to know where you were.
[The Man:] Like who?
[The Boy:] I dont know.
[The Man:] Like God?
[The Boy:] Yeah. Maybe somebody like that. (336.1-336.15)
It's also worth noting that later in the novel The Man uses the flarepistol as a weapon. A device used for signaling (read: language/communication) becomes a nasty weapon.