New Delhi, Dec. 26 -- Waymo is deploying a fleet-wide software update after a major power outage in San Francisco revealed a critical limitation in its autonomous driving system. The blackout forced the Alphabet-owned company to temporarily halt robotaxi operations as self-driving vehicles struggled at darkened intersections across the city.

The weekend outage, caused by a fire at a PG&E substation in the South of Market area, knocked out electricity to nearly one-third of San Francisco. Traffic signals went dark across large parts of the city, triggering severe congestion during one of the busiest shopping weekends of the year.

At the peak of the disruption, roughly 30% of the city were without power, with traffic lights failing from d...