Elon Musk tweeted Sunday that his underground transportation project will open its first test tunnel in December in Hawthorne, Calif.

If successful, the tunnel would demonstrate the viability of Musk’s plans to build a new type of high-speed public transportation system whose station elevators could be built into a city’s existing infrastructure.

Musk, a space and electric-car entrepreneur, has pledged to complete the test tunnel by the end of the year. Through his tunnel-digging firm, the Boring Company, Musk envisions the construction of a zero-emissions underground transit system that transports people using high-speed electric sleds. A bank of elevators would take pedestrians, cyclists or cars from street-level stations to underground tunnel stops.

Musk said on Twitter that the Boring Company will hold an opening event to showcase the test tunnel on the night of Dec. 10. The company will then offer free rides to the public the next day, he said, allowing passengers to travel as fast as 155 mph. The Hawthorne-based test tunnel begins at a SpaceX parking lot and continues underground for up to two miles, the Boring Company said.

Musk’s goal in designing an underground tunnel system is to “solve the problem of soul-destroying traffic,” the Boring Company says on its website. By lowering the cost of building tunnels and providing a transit alternative to driving, the company would help alleviate city congestion and offer high-speed transportation options across the country. One of the proposed routes would take passengers from the District to New York in under 30 minutes.

In addition to the testing tunnel and the East Coast route, the Boring company lists an underground tunnel project in Chicago.

The Boring Company also aims to enable ultra high-speed transit through the use of pod-and-tube transportation systems, known as a Hyperloop. For longer routes, this vacuum-based system could move people and cars without wind resistance, with pods traveling faster than 600 mph. Companies developing the technology include Virgin Hyperloop One and Hyperloop Transportation Technologies.

While Musk is known for his towering ambition and zealous breakthroughs, the tech billionaire also has a reputation for overpromising on meeting benchmarks. Using the phrase “Elon time,” critics of Musk have pointed to his history of missing deadlines or setting unrealistic timelines, most notably with the slow production Tesla’s mass-market Model 3.

“Dec 10 in real time or Elon time?” one Twitter user asked Musk about the test tunnel unveiling. “I think real,” Musk replied.