BORING: Elon Musk and Gayle King test drive the tunnel he hopes will solve L.A. traffic.

“Unless we can make tunnel digging at least 10 times cheaper, then digging tunnels will not be an effective means of alleviating traffic. It’ll just cost too much,” Musk said.

Musk’s vision depends on him being able to do it all faster and cheaper than current industry standards. While modern subway tunnels in Los Angeles cost around $900 million per mile, he says he built his for about $10 million. One way he saved money: he literally made it dirt-cheap.

“When digging tunnels…it’s quite expensive to have all this dirt trucked off somewhere. And we’re like, well, why don’t we try to use that dirt for something useful? So we are creating bricks on-site…and you can pick ’em up for, they’re very cheap; 10 cents a brick,” he said.

That’s quite clever, but this is brilliant:

At only 12 feet in diameter, it’s much more claustrophobic than most transportation tunnels. According to Musk, cars will be able to travel up to 150 mph in the tunnel but must be on autopilot.

“Because the autopilot has radar and cameras that will automatically slow you down before you impact another car…you would only be allowed to go through the tunnel on autopilot,” Musk said.

As tech blogger Dave Mark noted, “Only properly outfitted cars will be allowed in these tunnels. Someone who can build such cars efficiently can make a lot of money.”

Someone like Elon Musk.