Solutions like this are classic examples of tech-rich people thinking they have all the answers when there's a whole bank of qualified specialist people already working in that field who know what's really needed to fix the problem but have only been stymied by politics.
If traffic is driving Musk nuts then the solution is not to find innovative new ways to handle more traffic. The solution is to ask why is traffic so bad in the first place.
Recommended reading: The Death and Life of Great American Cities by Jacobs
Or if that's too heavy, try Suburban Nation: The rise of sprawl and the decline of the American dream.
Only then will you come to see the culprit: Single Use Zoning, aka the BANANA (Build Absolutely Nothing Anywhere Near Anything) rules. Single-use zoning forces everybody to make several car journeys just to get through a typical day. Going to work? Car. Going out for lunch? Car. Going home form work? Car. Need to go out for a bottle of milk and postage stamp? Car. Going to a movie? Car.
No bloody wonder the place is flooded with traffic. You try to build a city around the automobile and it becomes a hostile environment for pedestrians and cyclists. You try to widen roads to accommodate more cars and the laws of induced demand kick in, resulting in even more traffic and roads as choked as they were before.
Learn a few things about urban planning, Elon. Don't arrogantly assume that you're the first person to want to address this problem. Smart growth and sustainable, walkable, transit-oriented development is a far better solution than drilling holes in the ground and cracking puns about the word "boring." It requires years of tedious work and politicking to build support for smart growth. A city is not a private company with which you can do what you like. There are elected councils, public advisory committees, public hearings, tax implications, and all manner of complex bureaucratic hoops that you have to jump through to fix these things.