Elon Musk’s The Boring Company tested Full Self-Driving, an advanced driver assistance system developed by Tesla, in a tunnel that connects the Las Vegas Convention Center to several nearby hotels, Fortune reports.

Steve Hill, CEO of the Las Vegas Convention and Visitors Authority, told the outlet that The Boring Company has been testing the software on Tesla vehicles for several months with only safety drivers on board. But Hill said widespread application of the technology remains "out of reach."

The Tesla Tunnels have been open for about four years, and currently they only serve a small area under and around the convention center, although The Boring Company plans to expand them throughout Las Vegas.

Although Tesla recently launched a small-scale, invitation-only self-driving taxi service in Austin, Texas (and launched a similar small ride-hailing service in San Francisco), and Musk has touted how good the technology is, Hill said safety drivers still have to intervene "regularly" and take control of the vehicle.

In theory, simple tunnels should be easy for Tesla's self-driving software to solve. The biggest challenge may be guiding passengers to and from the various subway stations. Hill said the Boring Company tunnel's colored lights and semi-smooth rock walls have been challenges that driver assistance systems have to deal with, as cars "get into some difficult spots."