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.