HAWTHORNE, Calif. — Fed up with Southern California vehicle snarls, Elon Musk set out to solve the persistent urban irritant: the traffic. But rather than build atop the highway system, where his Tesla cars travel, or in the sky, home to his SpaceX rockets, he sought an answer under his feet: tunnels.

“I said, ‘What if we go down instead of up?’” Mr. Musk told Mayor Eric Garcetti of Los Angeles during a recent public discussion. “I’ve lived in L.A. now since 2002. Traffic has gone from bad to horrific back to bad.”

On Tuesday, Mr. Musk unveiled the first mile-long stretch of his underground vision of a transit system in Hawthorne, a suburb of 90,000 people about 15 miles southwest of Los Angeles. It is the home of both SpaceX and his tunneling enterprise, called the Boring Company.

But the promotional event, which attracted hundreds of people who lined up to see the tunnel, fell short of earlier promises of a system that could transport up to 16 people at a time in electric-powered pods. Mr. Musk said he had abandoned that concept in favor of a system using more conventional passenger vehicles.