Traffic jams with human drivers happen spontaneously when the cars get close enough that overcompensation turns into a negative spiral.
The roads are already there, electric cars are quiet, road trains are even more efficient.
I am no fan of American city design, having half encircled Davis CA on foot, having found the suddenly-terminating sidewalks in Salt Lake City, and having walked the really boring route from San Jose railway station to the hotel in the top left corner of the Apple Maps icon.
(NYC is surprisingly pedestrian friendly though).
But:
1. Musk doesn’t live in Europe, he set up TBC while stuck in a traffic jam in Los Angeles.
2. Solid car trains moving at full speed (whatever that is for the road, 20 kph for residential or much higher than the current rules for highways) use less land area than any other traffic, so some lanes can become bicycle only or whatever.
3. They can “perfectly” obey traffic lights and give way to pedestrians
4. Raised walkways are cheaper than tunnels
You might be right to do that — I’m no rocket scientist — but the target price would get a million people there for $200 billion, not your $5 quadrillion.
Yes, I also expect the first few thousand to be in easily damaged domes. I also expect them to bring some of The Boring Company equipment with them, as it is part-owned by SpaceX and the stated raison d'être of TBC being pointless in a future of fully self-driving cars.