Self-driving taxis operated by Waymo came to a standstill during a major power failure in San Francisco last weekend, blocking traffic and leaving passengers stranded. The company announced Tuesday it will update its automated software to better manage similar situations in the future.

Waymo vehicles froze at intersections across the city on Dec. 20 when traffic lights went dark following a fire at a PG&E Corp. substation. Videos posted on social media showed several cars sitting motionless in the middle of streets with their hazard lights on, backing up traffic behind them.

The Waymo Driver system treats traffic lights that have gone dark the same way it handles four-way stops. But sometimes a vehicle will “request a confirmation check to ensure it makes the safest choice,” according to a company statement released Tuesday. The cars managed to deal with 7,000 traffic signals that stopped working during the blackout. However, the widespread outage “created a concentrated spike in these requests,” the company explained.

This surge in requests slowed down how quickly Waymo could respond to its stuck vehicles. The company stopped its service after city officials asked drivers to stay off the roads.

Massive outage disrupts holiday weekend with abandoned Waymo cars

The power failure affected roughly 130,000 customers at its peak. It shut down stores and disrupted public transportation during one of the busiest shopping weekends of the year. Many Waymo cars, which are part of Alphabet Inc.’s robotaxi business, were caught up in the chaos.

Michele Riva, 30, was riding home in a Waymo on Saturday evening when the lights went out. His car kept going through a less busy part of the city, even as people walked across the street. But when he was just a minute from his destination, the vehicle stopped at a “very dense intersection” where the traffic lights had gone dark. He got no warning.

“I stayed in the Waymo for a couple of minutes, just to see,” Riva, who works as an engineer in artificial intelligence, told Bloomberg. “The problem was that, at the beginning, there were a lot of people crossing the streets because there were no traffic lights. So I believe Waymo just didn’t know what to do.”

Riva tried reaching customer support for about three minutes while stuck inside the car. He gave up after the wait became too long, as the service was swamped with calls from other passengers. He stepped out and walked the remaining few blocks home.

PG&E, which blamed the blackout on a substation fire, started bringing power back Saturday evening. By Sunday morning, the utility said it had restored electricity for 110,000 customers, though 21,000 remained in the dark.

On Sunday, the Waymo app displayed a message telling some customers that service was on hold in the Bay Area. At least seven cities were affected.

Emergency software updates in the works

“Our teams are working diligently and in close coordination with city officials, and we are hopeful to bring our services back online soon,” a Waymo spokesperson said Sunday.

Riva tried to book another Waymo ride on Sunday before seeing the notice.

“At the end of the day, I know it was an unpleasant situation for the other drivers, but I believe it really was all about safety — I believe it’s better safe than sorry,” Riva said. “I hope they will account for that in the future, because it’s truly a good service.”

Waymo said it is now working to add more information to its software about the “context” of regional power outages. This should help the vehicles “navigate these intersections more decisively.” The company is also teaming up with San Francisco Mayor Daniel Lurie on emergency response plans and updating training for first responders.

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