Were they even trying to redesign a TBM? I think they were just using off the shelf boring machines at a smaller diameter (many tunnels one lane each) because building tunnels wide enough for a highway is exponentially more expensive.

The funny thing is, the actual industry is going in the opposite direction, because it turns out that a single big tunnel is in practice cheaper than a bunch of small ones. Conventional deep-bore metro lines consisted of two tunnels about 3.5 meters in diameter, but if you look at _current_ metro projects, the Dublin metro is using a single 10 meter tunnel, the newer Barcelona lines are using a singe 12 meter tunnel...

The expensive bit of building metros isn't actually boring the tunnel, generally, not anymore. It's everything around that; securing a route, the disruption while it's going on, etc etc. So the last thing you want is to have to do it twice.

Elon did some napkin math along the lines of “we will make tunnel boring ~10x faster by: 4x lower boring surface area, 4x faster boring because the industry are currently idiots not operating their machines at the limits that physics dictates”. (I can’t remember if there was actually another factor of 2 in there)

Making a much faster TBM was absolutely part of the initial plan.

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