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.
The irony is if you designed a city with viaducts, the savings on ground disturbance and the extension in life for utilities (now high and dry instead of rotting in the dirt and corroding, being hit by fiber-seeking backhoes) pays for the viaduct system itself even if it costs tens of billions for a city.
When your domestic water system now lasts a century instead of 40 years, and leaks can be spotted and repaired from a catwalk, the savings compound over that same century. Apply that to power, gas, heating, cooling, telecom… Plus they stop hitting each other any time you need to dig more than a foot. Now telecom will stop hitting water lines when they go to repair broken fiber that was hit by a new construction excavating a foundation.
A 40 year buried power lifespan that cost $5 billion to install for a city means each year you need to replace 1/40th or your power cables and would annually spend 1/40th of $5 billion, or 125 million.
Those same cables in a utilities rack within a city viaduct system might last 2-3X as long since they’re dry, don’t move with frost heave, don’t experience being driven over by fully loaded semis, aren’t at risk of being hit while repairing something else… They also cost a fraction due to no ground disturbance being needed. It’s the same cost as installing power around an industrial plant in cable trays.