"It shall be called 'The Boring Company.' Boring, it's what we do."

Brunel had his ships. Trump had his walls. And now Musk wants to make... tunnels, tunnels under cities to reduce traffic congestion and make the world a better, cleaner, less rage-filled place.

Over the weekend, probably while sitting in traffic behind the wheel of an autonomous Tesla, Musk tweeted: "Traffic is driving me nuts. Am going to build a tunnel boring machine and just start digging." An hour later, probably while still sitting in traffic, giving him plenty of time to think of a witty pun, he declared: "It shall be called 'The Boring Company.' Boring, it's what we do."
And finally, an hour after that, just in case any of us were foolish enough to think the billionaire multi-CEO was joking, Musk said, "I am actually going to do this." He also changed his Twitter bio to include "Tunnels."

Elon Musk ✔ @elonmusk
Traffic is driving me nuts. Am going to build a tunnel boring machine and just start digging...
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Elon Musk ✔ @elonmusk
I am actually going to do this

So, unless Musk was suffering from a prolonged bout of entrepreneurial road rage, we now know roughly how long it takes a pedigree industrialist to pick a new disruptible domain: two hours, give or take.

Tunnels are indeed a pretty good solution for traffic congestion, though they take a long time to build, and the construction usually causes a huge amount of disruption above ground—especially if those tunnels are being built in a metropolitan area, which is where you'll find most of the world's congestion.

Depending on the setting, it can be very difficult and expensive to build tunnels as well. Cut-and-cover—where you dig up an existing road, build a tunnel, and put the road back—is the only "cheap" tunnel building method, but it's so incredibly disruptive that most tunnels nowadays are built at deeper depths by automated tunnel boring machines (TBMs). Cost-wise, you're looking at about £1 billion per mile for TBMs: London's Crossrail, with 13 miles of new tunnel, will cost around £15 billion; Manhattan's second avenue subway line, with 8.5 miles of new tunnel, will cost about $17 billion. The costs are much lower if you just want to bore through a mountain—the just-completed 35-mile Gotthard Base Tunnel through the Alps in Switzerland cost a mere £10 billion (and took 17 years to build!)—but I doubt Musk has those kinds of tunnels in mind.

https://youtu.be/z38JIqGDZVU

Working in three dimensions is certainly one way of circumventing the density limitations of planar infrastructure. Current technology would allow us to build dozens of crisscrossing subterranean tunnels at different depths, massively increasing the throughput of our roads and railways. But, except in a few cases—usually following a war—governments just aren't interested in spending that much on infrastructure.

Unless, of course, you're living in the US, where President-elect Donald Trump recently signalled that he wants to spend $1 trillion making roads, tunnels, bridges, and airports great again. And last week, Trump asked Elon Musk to be a strategic adviser. Hmm.

We can increase the throughput of our infrastructure in other ways, of course—and perhaps unsurprisingly, Musk has his finger in those pies as well. Autonomous cars and trucks, with vehicle-to-vehicle networking, will let us pack vehicles much closer together—though we may have to outlaw non-self-driving vehicles first. And then there's entirely new modes of transport, such as the Hyperloop, that would reduce the number of people using roads.
Fun fact: Did you know that Musk also conceived the Hyperloop while sitting in traffic outside Los Angeles?