It was common knowledge what an overpass was—that is, what they used to be. Vehicles once sped across them, taking people over buildings and streets to the hearts of neighbourhoods. People knew this. They saw vehicles, cars and trucks and panel vans, at the museum for a suggested donation. They could also see them, rusted to anonymity, in any corner of the Five Cities.
People knew about overpasses the same way they knew that thirty miles west was where the good wine came from, hills rolling with vineyards instead of houses and cul-de-sacs. They also knew by the same means that past the vineyards was a thick forest and beyond that, ocean.
A Big Horrible Thing had happened, half gradually, half suddenly. It brought the world (or at least this part of it) back to the crossroads of the pre-industrial era. It had not, however, made idiots of those remaining. They still had maps and endless books. There were still those who had taught and doctored, travelled and been born somewhere else.
So, when it was all over, they found a solid well of knowledge to draw from. They’d been very lucky overall. The Big Horrible Thing had not considered them very important and left mostly intact the area that became the Five Cities.
After a suitable period of mourning and madness, the people rebuilt and modified, finding that they—outside of some unpleasant instances—had adapted rather nicely. Children were taught history and an attempt was made to instill them with the same respect for knowledge which had been the privilege of their parents, more or less.
It was not strange then, that (years and years later) the first houses built on an overpass reminded people of the Old London Bridge. Nor was it surprising, after this realisation, that they took great precautions against fire. The people of the Five Cities were whimsical and flippant, not stupid.