Hey @Jorg1w, thanks for the follow request! Unfortunately there's not enough vibe on your profile to check, so...
If you could snap your fingers and replace cars with any other existing mode (or two) of transportation—complete with all the infrastructure needed—what would you choose?
*note: it doesn't *have* to be practical.
Since Seattle is hilly and has a big North-South ridge, I'd go for a fast rail down the middle, and hanging gondolas from the train stations down into all the East–West neighborhoods. The rest would be mostly footpaths.