On Saturday, Elon Musk — the restless billionaire behind Tesla, SpaceX, and SolarCity — was stuck in annoying traffic and mused that one solution would be to drill more tunnels. All he’d need, really, is a giant machine…

Traffic is driving me nuts. Am going to build a tunnel boring machine and just start digging... — Elon Musk (@elonmusk) December 17, 2016

It shall be called "The Boring Company" — Elon Musk (@elonmusk) December 17, 2016

Boring, it's what we do — Elon Musk (@elonmusk) December 17, 2016

I am actually going to do this — Elon Musk (@elonmusk) December 17, 2016

Everyone’s trying to figure out whether Musk is genuinely serious about starting a brand-new tunnel-boring company or not. Who knows! But tunnels are fun to think about, so why not pretend he is serious and think through how this might actually work....

Drilling tunnels is hard, though that’s not the main obstacle here

One obvious hitch to Musk’s scheme is that some cities are already trying to dig new tunnels to accommodate traffic — and it’s far from easy! As Steve Hanley points out, there’s a lot of existing infrastructure buried in the ground beneath cities, from water mains to electrical cables. And the tunnels themselves often need to be reinforced. That makes tunneling slow, difficult, and expensive work.

Seattle is a cautionary tale here: Since 2013, the city has employed a massive 57-foot-diameter boring machine named Big Bertha to drill a 2-mile highway tunnel beneath the downtown area. The machine looks like this:

Yet six months after work began, Big Bertha broke down after overheating. Drilling finally resumed in late 2015 — but then had to stop again after a sinkhole opened up near construction. Seattle’s $1.4 billion tunnel project has faced significant cost overruns, though it appears to be making progress again.*

Perhaps Musk can improve on Big Bertha. But that brings us to an even deeper problem with his idea. Building more tunnels is just not a good way to alleviate traffic congestion. In fact, it would likely do the opposite.

The "fundamental rule" of traffic: building new roads just makes people drive more

In January 2016, during SpaceX’s Hyperloop pod design competition, Musk explained why he thought tunnels could help alleviate traffic:

It’s a really simple and obvious idea and I wish more people would do it: build more tunnels. Tunnels are great. It’s just a hole in the ground, it’s not that hard. But if you have tunnels in cities you would massively alleviate congestion and you could have tunnels at all different levels — you could probably have 30 layers of tunnels and completely fix the congestion problem in high-density cities. So I strongly recommend tunnels.

Except economists and traffic experts have been studying this issue for a long time and they’ve found the exact opposite. When cities add new roads to a congested area, it usually doesn’t alleviate congestion. Instead, it just induces more traffic, as people take advantage of the added road space to drive more.

In fact, this pattern is so consistent across cities that economists Matthew Turner and Gilles Duranton call it the "fundamental rule" of road congestion: Adding road capacity tends to increase the total number of miles traveled by all vehicles. (Obviously in theory you could build such a ridiculous number of roads that there’s no longer any congestion, but in practice, the “fundamental rule” has held — more roads have historically led to more traffic.)

It sounds counterintuitive at first, but there’s a logic too it: People aren’t typically charged every time they drive on a road. So if you offer more of this valuable resource that’s free to use, people will use more of it. Economists call this phenomenon “induced demand”; it also shows up when cities offer free parking.

Granted, there can still be good reasons for fast-growing cities to build new roads. They just shouldn’t necessarily expect traffic jams to disappear as a result. Los Angeles got a firsthand glimpse of this after widening its I-405 freeway, a project that cost $1 billion. "The data shows that traffic is moving slightly slower now on 405 than before the widening," Turner told my colleague Joseph Stromberg in 2014.

So what does help alleviate congestion?

If cities really want to erase traffic jams, many transportation economists would instead recommend that they charge people to use roads when they’re crowded — a policy known as congestion pricing that has popped up in places like London, Singapore, and Stockholm.

Early research suggests that pricing really does cut down on traffic, as people decide to move their commutes to non-peak hours, shift to mass transit, or cut down on trips overall. It’s arguably even more effective if cities use the funds to provide alternative transportation options.

The downside is that congestion charging tends to be rather unpopular, since people don’t like it when they suddenly have to pay for something that used to be free. (It’s the same reason why checked-baggage fees on airplanes have incurred such a backlash.) So urban planners tend to favor building new roads and widening existing roads — or, in Musk’s case, new tunnels — even if the research suggests again and again that it doesn’t cut down on congestion.

Now, that doesn’t mean a tunneling machine would be useless! Remember, Musk also has plans to start colonizing Mars within a decade. And humans living on Mars would probably want to spend most of their time underground to avoid the higher levels of solar radiation that hits the planet. Hmmm.

* Correction: Seattle’s tunnel project costs $1.4 billion, not $3.1 billion as originally stated. The entire viaduct costs $3.1 billion, but the tunnel is just one component of that broader system.

Further reading

A few other good pieces on Musk’s tunnel plan from Alan Boyle and Steve Hanley.

As economist Edward Glaeser explains in this interview, the “fundamental rule” is likely to apply to self-driving cars, too. If they make driving easier and quicker, people are likely to take more trips — and the result could well be worse traffic. That said, he does note that it might prove easier to introduce a congestion charge to a brand-new technology like self-driving cars than it is to slap it on our current transportation system.

Watch: How highways wrecked American cities