Now that I live in a city with a good train system (Chicago), a plan to alleviate traffic in large cities by boring tunnels for more cars sounds incredibly inefficient, and clearly driven by Musk's incentive to sell cars.
There's no way to alleviate traffic in large cities with cars; it just isn't possible as long as you require everyone to have a car. Making roads wider just brings more cars as people move farther away and have to drive more, making more traffic and more calls to make more and wider roads, etc.
And you would have to create parking space for all those cars. Cities have very expensive land. I suppose you could turn central park into a parking lot.
Even that wouldn't work, because then how do you get from Central Parking Lot to the building in Manhattan that you want to go to? To many NYC car-friendly, you need to demolish most of the buildings so you can put huge parking lots around the ones that remain. Then the roads won't be wide enough, so you'll need to demolish more buildings to make wider roads. Pretty soon, there won't be anything there besides roads, parking lots, and a handful of buildings, and no one will want to go there, they'll want to go someplace else that has more buildings and things to do, and then the traffic in that place will be too much and people will complain.
Edit: To make NYC car-friendly...
Don't get me wrong, I live in the Loop and don't own a car, but when I rent one, Lower Wacker Drive is kind of magic... (I think it helps that GPS doesn't work down there so lots of people are scared to use it).