I interpreted the scene with the highway and the skeleton as a subtle bit of foreshadowing. It'll take some inference to get there, so bear with me if you will.
The highway is the progress of history
Pavel says as much in the given quote: "The highway was anisotropic, like history. You weren’t supposed to go back."
However, going back through history is exactly what Anton does. He immerses himself in the society of Arkanar, which the people of Earth consider to be at an earlier point of historical progression. More than Don Condor or Pavel himself, Anton goes native. He has real friends among the natives, like Baron Pampas. He falls in love with Kira, a native woman. He even adopts the passionate hatred of the natives, abandoning the more appropriate dispassion and pity, as he himself notes in Chapter 5:
Why? What has happened to me? Where did it go, my nurtured-since-childhood respect and trust in my own kind, in man— the amazing creature called man? Nothing can help me now, he thought in horror. Because I sincerely hate and despise them. Not pity them, no— only hate and despise. I can justify the stupidity and brutality of the kid I just passed all I want— the social conditions, the appalling upbringing, anything at all— but I now clearly see that he’s my enemy, the enemy of all that I love, the enemy of my friends, the enemy of what I hold most sacred. And I don’t hate him theoretically, as a “typical specimen,” but him as himself, him as an individual.
And a little further on in the same long monologue:
I came here to love people, to help them unbend, see the sky. No, I’m a bad operative, he thought remorsefully. I’m a no-good historian. When exactly did I manage to fall into the swamp that Don Condor was talking about? Does a god have the right to feel anything other than pity?
Even though Anton has not literally traveled through time, he's absorbed himself so thoroughly in historical conditions that he's started to adopt historical ways of thinking—hatred, vindictiveness—even while he keeps his values. Anton still values the things he was taught to value on Earth, like learning, curiosity, discovery, and peace, but he despises those who don't embody those values, instead of just condescendingly pitying them and hoping for the enlightenment of their descendants. It becomes so bad that he fantasizes about killing Don Reba and starting his own rebellion in Chapter 3:
But the very thought that thousands of others— maybe less talented but also honest and truly noble people— were fatally doomed caused an icy chill in his chest and an awareness of his own vileness. Sometimes this awareness became so acute that his mind would become clouded, and Rumata could almost see the backs of the gray bastards illuminated by lilac flashes of gunfire, and Don Reba’s eternally insignificant, pale visage contorted with animal terror, and the Merry Tower collapsing on itself. Yes, that’d be sweet. That would be actual work. An actual macroscopic impact.
Hard to Be a God (Rediscovered Classics) (p. 77). Chicago Review Press. Kindle Edition.
He also thinks how he envies Arata, the consummate rebel who struggles against the crown:
Arata was the only person here for whom Rumata felt neither hatred nor pity, and in his earthling’s dreams— the feverish dreams of a man who had lived for five years surrounded by stench and blood— he often imagined himself as such an Arata, having received the high right to murder the murderers, torture the torturers, and betray the traitors for having passed through all the hells of the universe.
Ibid pp. 213-214
The only thing that ends his fantasies is foreknowledge of the rapine that would follow such an uprising:
Then the inevitable. Bloody chaos in the country. The surfacing of Waga’s night army, ten thousand thugs excommunicated by every church— rapists, murderers, and sadists; hordes of copper-skinned barbarians descending from the mountains and destroying everything that moves, from newborns to the aged; huge crowds of peasants and townspeople, blind with terror, fleeing to the forests, mountains, and deserts [...]
Ibid, p. 77.
Neither Anka nor Pavel follow Anton up the highway. Neither feels the same temptation he does: Pavel manages to maintain a proper distance from his work, while Anka stays on Earth, ever the cautious one.
The skeleton represents figures like Don Reba who obstruct the progress of history
In Chapter 10, Anton sums up Reba thusly:
We’ve been racking our brains, vainly trying to squeeze the complicated, contradictory, enigmatic figure of our eagle Don Reba into the ranks of Richelieu, Necker, Tokugawa Ieyasu, and Monck, and he turned out to be a petty hoodlum and an idiot! He betrayed and sold out everyone he could, got tangled up in his own schemes, got scared to death, and ran to the Holy Order to be saved. In half a year he’ll be slaughtered, and the Order will remain.
Ibid, pp. 220-221.
Don Reba's determination to fight, to impose his own will on the country, led to his own death; he purposefully chained himself to his weapon by refusing to stop weaving schemes within schemes, trying to improve his own lot; but in the end, chained to the gun, he couldn't stop fighting, and he had to take things to their bitter conclusion. His legacy ended there as he sank into the ground, pulled down by his weapon, the Holy Order.
The bridge represents the dark age that Arkanar has fallen into by the end of the book
Since the highway is the progress of history, the bridge was what allowed civilization to move smoothly from one age into the next. But the bridge was blown up, probably in the same battle that the fascist died in.
Without the bridge, making progress up the road is difficult. But it's not impossible to ford a river or wade through a swamp or trundle through a valley. It's slow, dangerous, and many more will die than would have walking across the bridge. But some can still make it to the other side.
This is Arkanar at the end of the novel: their society's bridge to the next era of history has been blown up thanks to Don Reba. People who would have survived into the next age will now die, and the path for those who survive will be much harder than it would have been. But because Anton and his colleagues managed to save at least a few of the people who will usher in the next era, it's still possible for their society to finally come out on the other side and continue down the road of history.
