Underground housing, underground businesses, etc. Would that be better for the environment + possibly save on energy costs? Also possibly safer in certain scenarios like tornadoes etc.
Potential issues that immediately come to mind are ventilation, earthquakes, and flooding. But it’s not like underground dwellings/basements/etc. aren’t a thing, so maybe those issues have been addressed in ways I’m not familiar with.
Afaik you build buildings on raised foundations and the viaduct decks span the gap between buildings creating a raised “ground floor” above the actual dirt. They eventually do wear out given enough loading cycles accumulating fatigue in the metal reinforcement, but can last a hell of a long time if you keep heavy vehicles off of them.
In an ideal world the viaduct top is for pedestrians or bicycles only, and there’s enough space underneath for logistics to supply businesses from loading docks at their basement. Overhead LRTs would be a natural pair with viaducts since you can just build the LRT piers to put their load path into the viaduct columns (which you also engineer to be larger.) that way you can separate all traffic types by verticality instead of all sharing the same grade.
The big benefit there is the viaduct deck doesn’t fatigue hardly at all. Maybe emergency vehicles allowed up on the deck? Otherwise it’s just bicycles or pedestrian traffic.