This short story has a dual theme and is a most interesting study of a much anticipated event during the time in which the story is set. The time period of the setting is integral to understanding the themes. Hernando's life in the jungle gives very little information from which to discern the time period, but the "swift long low convertible cars," like the Chrysler New Yorker Convertible, for instance, that flee up the highway northward set the time period around 1950. "The Highway" was published in the collection, The Illustrated Man, which was published in 1951.
After the atomic bomb was dropped on Hiroshima, the Western world was gripped by the dark fear of all-out atomic warfare. School children practiced pointless drills to "protect" themselves in the event of an atomic attack. Families dug out or built up atomic bomb shelters in their back yards or in their cellars. TV commercial public service announcements warned in chilling images about the dangers of and what to do in the event of atomic attack. The fear of atomic war was palpable--almost tangible.
It is in this time period that the story is set and this time period informs the significance of the dual themes. The first theme, which centers around those fleeing civilization, is the fear and destruction of atomic war. Bradbury gives an almost surreal glimpse into this fear of atomic war as he briefly describes the fleeing occupants of the cars as having something unidentifiable in their faces:
Five hundred, a thousand cars passed, and there was something in the faces of all of them. But they moved too swiftly for him to tell what this thing was.
The second theme, which centers around Hernando, is the contrast between civilization and the world. Ironically, Hernando's last words in the story are, "What do they mean, 'the end of the world'?" Before the last Ford of the civilized towns rushes away up the highway, the man tells him that the "atom war" had come and that it is the end of the world.
After this final car drives away, the smell of the jungle after the rain storm that starts the story wafts up to Hernando. He picks up his plow and prods his burro to work with the familiar cry. As his life continues as always beside the now deserted highway, he wonders what will be destroyed as his "world" consisting of the river, jungle, soil, and growth and life remains unchanged.
[Recall that in the 1940s and 50s, far less was known about the global contamination and destruction of atomic radiation. Based on what is now known, the continued health and quiet life of the jungle and Hernando would be improbable.]