Social media users posted videos of Waymos as they encountered traffic lights that were off. Some cars’ hazard lights blinked and they abruptly stopped in place, failing to cross the intersection. Others stopped in the middle of the intersection, forcing other cars to swerve around them.

The power outage affected 130,000 homes and businesses in San Francisco, nearly one-third of the customers served by Pacific Gas and Electric Co. It was caused by a fire at a power substation, officials said. On Monday, the utility company was still working to restore power to thousands of customers.

Waymo operates hundreds of robotaxis in San Francisco, but it wasn’t clear how many cars were on the road at the time of the outage. The company paused service Saturday evening and resumed it Sunday afternoon.

The road-blocking problems that prompted Waymo to suspend its service during the weekend power outages revived concerns that city officials raised about the robotaxis periodically coming to abrupt and inexplicable stops before California regulators approved them as a commercial service in August 2023.

Tyler Cervini, who lives in the Mission District, said he was calling an Uber to bring him to the airport since his train station was not operating due to the outage. At the traffic light outside his apartment, there were five Waymos crowding the intersection, he said.

He got into his Uber right outside where all the Waymos were, but his driver “had to swerve through them to pick me up,” Cervini said. “He seemed extremely frustrated by what was going on.”

Waymo said that its vehicles are designed to treat nonfunctioning traffic signals as four-way stops, but the scale of the outage created unusual conditions.

“While the failure of the utility infrastructure was significant, we are committed to ensuring our technology adjusts to traffic flow during such events,” a Waymo spokesperson said. “Throughout the outage, we closely coordinated with San Francisco city officials.”

The company said most active trips were completed before vehicles were safely returned to depots or pulled over.

Philip Koopman, professor emeritus at Carnegie Mellon University and expert on self-driving vehicle safety, said the scale of the traffic disruption was concerning. Autonomous vehicles are generally programmed to come to a stop if they are unsure or confused on what to do and ask for remote assistance, he said.

Koopman said it did not appear to be a software failure in the cars themselves, but an “operational management failure” where the company did not have the capability to deal with so many robotaxis needing assistance at once.

Waymo should have suspended service earlier — as soon as their vehicles started having issues, he said.

“If you have thousands of robotaxis that stop, you have a problem,” he said. “What if this had been an earthquake? You would have thousands of robotaxis blocking the road.”

Waymo, which started as a secret project within Google in 2009, has steadily expanded its operations in San Francisco while also introducing its robotaxis into other California cities such as Los Angeles and San Jose, in addition to other U.S. markets in Texas, Arizona, Florida and Georgia.

In the months leading up to the approval from the state’s Public Utilities Commission, San Francisco’s transportation and fire department leaders flagged dozens of reports about robotaxis coming to standstills, blocking traffic.

Besides inconveniencing other drivers trying to get to their destinations, the road-blocking robotaxis were viewed as a possible impediment in life-threatening emergencies when firefighters and police officers were responding to calls for help.

Waymo’s fleet of robotaxis is on pace to complete more than 14 million rides this year, more than tripling from last year, according to the company.

California is considering expanding approval for heavy-duty autonomous trucks and vehicles carrying up to 15 passengers to operate, a move opposed by unions representing truck drivers.

Shane Gusman, director of Teamsters California, called the Waymo disruption “a clear warning that turning our roads and lives over to autonomous vehicles is premature and dangerous.”

“We live in a state where blackouts, wildfires, floods and earthquakes affecting power and roadways are all too common,” Gusman said in a statement. “AVs stalled in streets and intersections threaten the safety of AV passengers, and others on the road, and inhibit emergency response when we need it most.”