I wonder if this would be possible if you could abstract away enough detail when necessary, and bring it back when zooming in. For example, when you zoom out from a city, the server stops calculating individual vehicles' paths, and instead models the city as a node with x cars flowing in via I-69, y cars flowing out via I-01, etc. Using this you could even abstract away entire planets.
Then when the player zooms in again, individual roads can still be adjusted, which simply changes the formula describing the city's traffic flow.
I'm not sure how well that could work when I want industry to have the same depth as C:S. Like, if a commercial building is expecting a shipment from a specific industrial building two cities away, the location of that truck needs to be tracked through its entire journey.
I suppose it could be possible for a server to track something like "The last 10 trucks to enter my area at location A and leave my area at location B took an average of C minutes, so just assume any path finding through me will take C minutes and don't track the individual trucks"
But then what happens when some zooms into that server's area while a truck is partway through its course? How do I simulate a traffic jam occurring after a path has been planned if I'm abstracting the individual vehicles away?
Then when the player zooms in again, individual roads can still be adjusted, which simply changes the formula describing the city's traffic flow.