Considering it costs $1 billion per mile to dig L.A.'s subway tunnels, how does Elon Musk plan to build a series of car-carrying tunnels with out breaking the bank?

Elon Musk is serious enough about his plan to build tunnels for cars below L.A. that he has at least given thought to the huge hurdles his plan will have to overcome—namely, the cost of engineering such an endeavor. Ruth Reader of Fast Company writes that the entrepreneur plans to dig the tunnels in a much more cost effective manner, "correcting inefficiencies" in the process.
(Musk) plans to make the diameter of the tunnels only 12 feet wide, much smaller than the typical 26-28 feet. He also says that a machine that tunnels and reinforces continuously will allow the burrowing process to go more quickly—especially if he were to increase the power of such a machine.
Elon Musk's discussion of the tunnel project and the Hyperloop transportation system from the TED2017 conference can be viewed online here.
FULL STORY: Elon Musk Shows Off A “Boring” Tunnel Network To End L.A.’s Traffic Jams

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Analysis: Cybertruck Fatality Rate Far Exceeds That of Ford Pinto
The Tesla Cybertruck was recalled seven times last year.

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