Not having read the book either I can only relay a few thoughts that occurred to me after viewing the film.
Possible mild spoiler warning in final paragraph
I'm probably reading too much (or inadequately little!) into this, but The Road shares a common factor of lot of recent American science fiction cinema, the threat of sudden catastrophic breakdown down of social and cultural values in the wake of some overwhelming disaster, partly in reaction to 9/11 but possibly also due to anxieties about modern society. Perhaps even a fear that civilisation is far less resilient than we like to imagine. What is it after all but a series of attitudes and technical know how, largely inherited from the sum of all previous generations and which in the most extreme scenario could conceivably be lost in a single generation.
War Of the Worlds was an interesting though frustrating example. The central character had just enough specific know how to survive the initial disaster, though barely enough personal character to keep his family together before the event. He is almost unable to sing a lullaby to his daughter in one of the scenes afterward, let alone set about recreating even the most basic functions of a working society should he survive. In some of the more vicious crowd scenes Spielberg seems to partly suggest that the veneer of civilisation is perilously thin, even though the ending is remorselessly upbeat.
In The Road and in War Of The Worlds, both protagonists are isolated more through their own fears, than selfishness, greed and/or ignorance, and where survival alone is almost impossible and ultimately futile, survival together is just as difficult, fraught with distrust and fear.
In The Road, because the nature of the disaster is never explained we must assume that it's of no importance to the story, we don't need to be sidetracked by the how or why. What we do know is that all resources are scarce, infrastructure has collapsed, and isolated bands of survivors mostly fight each other to possess what little remains. Society is not only breaking down, it is gone.
The father is trying to protect his son while they drift through the desolation, in the wake of further personal tragedy for them, looking for somewhere better, discovering nothing but horror piled upon horror as some of the almost tribal stragglers they encounter have resorted to harvesting the most easily available source of protein remaining.
As they journey, the father is continually nagged by the sense that they are being pursued. The single most palpable element of the story is his continual and sustained state of fear which drives most of the subsequent events through his own actions in response. Only when the father dies does the boy rediscover a semblance of security, community, family and hope. A worthy though not really enjoyable film.
it bombed so bad) 
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