Self-Driving Cars Snarl San Francisco After Power Outage. Tesla Stock Hits Record.

Coming into Monday trading, Tesla stock was up 19% this year. (Justin Sullivan/Getty Images)

Key Points

• A power outage in San Francisco caused Waymo’s robo-taxis to stall, while Tesla’s robo-taxis were unaffected.
• Tesla’s CEO Elon Musk highlighted on X that Tesla’s robo-taxis continued operating during the blackout.
• Tesla shares rose 1% in premarket trading.

Tesla’s robo-taxi battle with Alphabet’s Waymo took an odd twist over the weekend.

A power outage in San Francisco on Sunday left about 130,000 homes and businesses in the dark. It also left Waymo’s robo-taxis seemingly stalled in the streets, waiting for traffic lights that weren’t available.

The situation might have been perceived as a win for Tesla. Its shares closed up 1.6% at $488.73, while the S&P 500 and Dow Jones Industrial Average rose 0.6% and 0.5%, respectively.

Tesla stock just missed closing at an all-time high. Its record close—$489.88—was reached this past week. Shares did hit a new 52-week high at 498.83, less than $2 away from $500 a share.

Waymo chose to pause its service during the blackout. The company said its cars treat nonfunctioning traffic lights as four-way stops, but the added caution worsened congestion, prompting the decision.

Still, Tesla CEO Elon Musk took the opportunity to point out on his social-media platform X that Tesla robo-taxis were unaffected by the power outage.

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Tesla is testing robo-taxis with safety drivers in the front driver’s seat in the city.

Tesla launched its robo-taxi service in Austin, Texas, in June with safety monitors in the front passenger seat. Waymo operates in five cities today, with no safety drivers or monitors, completing more than 450,000 fully autonomous cab rides a week.

To be sure, Waymo likely could have continued to operate. The majority of active trips were successfully completed before vehicles were safely returned to depots or pulled.

Tesla’s robo-taxi technology is built on its Full Self-Driving driver-assistance system, which will keep driving, paying attention to its surroundings, even when the lights are not working.

The incident highlights some of the differences between the two robo-taxi competitors. Waymo started as a robo-taxi service, mapping city streets, and using a suite of expensive sensors, including lidar, which is essentially laser-based radar. Tesla started with advanced driver-assistance technology for passenger cars that relies on optical cameras.

Both approaches appear to be working, but there are still many situations that are unfamiliar to the new technology, such as power outages.

The San Francisco outage won’t be the definitive battle in the robo-taxi wars. It is just one incident that demonstrates the technological approach of two leaders in the space.

Alphabet stock was up less than 0.1% in early trading at $307.28.

Write to Al Root at allen.root@dowjones.com