It took a blackout across a huge swath of San Francisco for Waymo to figure out that its robot system for handling such situations has a key weakness, which caused huge traffic problems on Saturday.

On Tuesday, Waymo divulged a clearer explanation for why its autonomous taxis, which are programmed to treat broken traffic signals as four-way stops, could not handle the mass shutdown of traffic signals and consequent traffic confusion on Saturday. And it's a prime example of where autonomous vehicles may fall short or become outright dangerous to humans in a variety of emergency scenarios.

In a post on the company's official blog, Waymo says, "The scale of the outage and the sheer number of disabled traffic lights were the primary contributors to city-wide gridlock. As signals went dark across major corridors, the resulting congestion required law enforcement to manually manage intersections."

But, the company admits that, "Navigating an event of this magnitude presented a unique challenge for autonomous technology." And, they explain, "While the Waymo Driver is designed to handle dark traffic signals as four-way stops, it may occasionally request a confirmation check to ensure it makes the safest choice."

That "occasionally" escalated into hundreds or more incidents, apparently during Saturday's outage event, and the company claims that while their cars "successfully traversed more than 7,000 dark signals... the outage created a concentrated spike in these requests."

So, this left Waymo staff overwhelmed, creating a backlog of these requests to take remote control of the cars and navigate through confusing intersections, and that, Waymo says, "led to response delays contributing to congestion on already-overwhelmed streets."

The Great San Francisco Blackout of December 2025 only lasted until Saturday night for some parts of the city, but for others it dragged on for three days, and PG&E will continue to have to deal with the fallout from that.

But in addition to local lawmakers once again questioning PG&E's death grip on the city when it comes to electrical power, there are also calls to rethink policies around autonomous vehicles, especially when emergencies arise. The question widely being asked now is, in the event of a major earthquake when emergency responders need to traverse the city to handle life-and-death situations, will the streets be clogged with confused robot cars again?

SF Mayor Daniel Lurie took credit for personally calling one of Waymo's co-CEOs, and telling them they needed to "get the cars off the road immediately" on Saturday. He said on Monday that while the company was "understanding... we need them to be more proactive."

In other words, higher-ups Waymo should have seen a massive blackout in the city and immediately thought, "Oh, we'd better pull cars off the street before this becomes a problem."

On Monday, SF Supervisor Bilal Mahmood vowed to get to the bottom of the situation, which may have prompted Waymo to be a bit more transparent. As Mahmood said, "Given my background in the artificial intelligence industry, I plan to ask the hard technical questions our residents want answers to so we ensure safety in our streets no matter the conditions."

Waymo said that it is making updates to its Driver software to address what it learned from Saturday's debacle. And, the company said, it will also be working to do more training with first-responders in cities where it operates so thaty no better how to interact with the cars when problems like this arive.

Awkwardly, Waymo's blog post says, "As we discover learnings from this and other widespread events, we’ll continue updating our first responder training."

Related: Supervisor Mahmood Calls For Hearings Into Waymo After Last Weekend’s Mass-Stalling Shambles

Image: walden_yan via Twitter