The December 20, 2025 San Francisco traffic collapse was caused primarily by a major power outage that disabled traffic signals across nearly one-third of the city. But Waymo’s robotaxis added a second problem: a surge in remote-assistance requests left some vehicles stationary in intersections and traffic lanes, including on routes needed by emergency responders.
The distinction matters. Waymos did not cause the citywide gridlock by themselves, but the company’s cautious fallback behavior and overloaded response operation made an already dangerous situation worse.
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The short version
- A fire at a PG&E substation triggered a widespread outage on December 20, 2025.
- Thousands of traffic signals went dark, disrupting drivers, transit and emergency operations.
- Waymo vehicles were designed to treat dark signals as four-way stops, but some situations prompted requests for confirmation from Remote Assistance.
- Those requests rose sharply around 5 p.m., response times lengthened and some vehicles remained stopped.
- Waymo paused service at approximately 5:55 p.m. and later announced software and emergency-response changes.
Waymo’s own regulatory filing says its vehicles contributed to congestion, while also describing the disabled signals and broader traffic breakdown as the primary causes of the gridlock. Waymo’s CPUC filing is the clearest source for that distinction.
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What happened on December 20?
A PG&E substation fire triggered an outage that affected nearly one-third of San Francisco. Traffic signals across a large area went dark, transit service was disrupted and ordinary traffic quickly became chaotic. San Francisco’s Department of Emergency Management urged residents to stay home while police and other officials manually managed intersections.
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Waymo vehicles encountered the same failed infrastructure as human drivers, but their operating system responded through a defined safety process. As the outage spread and traffic conditions deteriorated, the number of unusual dark-signal situations rose well beyond normal levels.
Waymo reported that its vehicles traversed more than 7,000 dark traffic signals during the event. That is the company’s operational count, not a claim that 7,000 different intersections were impassable or that every crossing produced a problem. Waymo’s account of the outage says the fleet was generally able to handle dark signals, but a concentrated spike in requests for additional confirmation created delays.
Timeline of the failure
| Time | What happened |
|---|---|
| Around noon | The outage and associated traffic-signal failures began affecting the area. |
| Afternoon | The outage expanded, transit was disrupted and traffic conditions worsened across affected corridors. |
| Around 5 p.m. | Requests for remote confirmation markedly exceeded normal volumes, according to Waymo’s CPUC filing. |
| Approximately 5:55 p.m. | Waymo paused service and began directing vehicles to pull over or return to depots in waves. |
| Evening | Emergency agencies continued responding to blocked routes, signal failures and unusually heavy call volumes. |
These timestamps describe different stages of the incident. The start of the blackout, the rise in Remote Assistance delays, the service pause and the eventual clearing of individual vehicles were not one instantaneous event.
Why did some Waymos stop?
A dark traffic signal is not necessarily an impassable intersection. Waymo says its vehicles are designed to treat a dark signal as a four-way stop and use the surrounding situation to decide how to proceed.
In some blackout conditions, however, the Waymo Driver can ask Remote Assistance for confirmation that proceeding is safe and appropriate. Remote Assistance is not the same as remote driving: the documented process describes people providing information or confirmation, not continuously steering the vehicle.
Under ordinary conditions, these requests are handled promptly because they are relatively uncommon. During the outage, demand arrived at a much larger scale. When confirmation took longer, some vehicles remained stationary rather than proceed without the required confidence.
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That created the central trade-off in the incident:
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- Traffic and emergency access: Waiting in an intersection or lane can block other road users and prevent emergency vehicles from getting through.
The documented failure was therefore not simply that the cars could not tell a traffic light was out. It was a fleet-scale operational bottleneck: a normally manageable exception became a simultaneous citywide demand spike.
How many Waymos were involved?
The strongest verified figure is 829 Waymo vehicles that operated for some amount of time within the outage-affected geographic area between noon and 11 p.m. on December 20. That does not mean all 829 vehicles became stuck.
San Francisco officials and reporting by the San Francisco Chronicle referred to nearly 1,600 Waymo stoppages. A stoppage is not automatically a unique vehicle. The figure may include repeated events involving the same car, and the public record does not establish a definitive fleet-wide count of uniquely immobilized vehicles.
Videos and photographs show visible examples, but they cannot establish how many vehicles stopped, how long each one waited or whether the same vehicle appeared in multiple recordings. Describing “1,600 frozen Waymos” would overstate what that number proves.
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No, not the citywide traffic disaster. The blackout disabled traffic signals across major corridors and created gridlock for human drivers, buses, emergency crews and Waymo vehicles alike.
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Yes, in a narrower contributing sense. Waymo acknowledged that some vehicles remained stationary while waiting for confirmation, adding concentrated blockages to streets that were already overwhelmed. The most accurate description is that the outage created the crisis and Waymo’s fallback and response bottleneck amplified it in specific locations.
That distinction also avoids assigning every traffic problem in the city to autonomous vehicles. Human drivers reacting unpredictably to dark signals, transit disruption, police controls and the loss of normal traffic flow were all part of the wider breakdown.
The emergency-response consequences
The incident became more serious than a collection of viral videos because city emergency records submitted to the California Public Utilities Commission described specific impacts on responders.
- A fire unit reportedly encountered a Waymo vehicle blocking traffic near the PG&E substation.
- An ambulance near Natoma Street and Seventh Street was reportedly delayed by a disabled Waymo vehicle. San Francisco Fire Department records submitted to the CPUC describe a delay of 40 minutes.
- San Francisco emergency-management dispatchers placed 31 calls to Waymo’s first-responder hotline between roughly 3 p.m. and 8 p.m. Staff reported spending a total of two hours and 36 minutes on those calls, much of it on hold.
- The San Francisco Fire Department received 801 calls in the 24 hours from 8 a.m. December 20 to 8 a.m. December 21, compared with a daily average of 507—a reported 58% increase.
These figures come from emergency-services records filed in the CPUC proceeding. They should be understood as reported operational evidence, not as a final regulatory finding that Waymo caused every emergency delay.
They do show why autonomous-fleet readiness cannot be judged only by whether an individual vehicle avoids a collision. A vehicle can be mechanically operational and still be operationally unusable if it cannot clear an emergency route or be reached quickly during a disaster.
When did Waymo suspend service?
Waymo said it paused service at approximately 5:55 p.m., as the outage and traffic conditions worsened. It directed vehicles to pull over and park appropriately, then return them to depots in waves so they would not further obstruct traffic or emergency vehicles.
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A service suspension does not instantly remove vehicles already stopped on the road. The company still had to locate vehicles, communicate with them, identify suitable places to stop and recover them without adding more congestion. That is why the timing and effectiveness of fleet recovery matter as much as the moment new rides stop being accepted.
What did Waymo change afterward?
In its December 23 account, Waymo said it would deploy fleet-wide updates that give the Waymo Driver more outage-specific context and make it more decisive at disabled signals where appropriate. The company also said it was revising emergency-preparedness procedures, improving first-responder coordination and expanding engagement and training with emergency agencies.
Those changes address several different layers of the failure:
- Vehicle behavior: better handling of dark signals and unusual intersection conditions.
- Fleet operations: a response system able to absorb a sudden, large increase in assistance requests.
- Emergency management: faster contact, clearer escalation and quicker removal of vehicles blocking responders.
Waymo’s announcement is not independent proof that the problem was fully solved. A software update may improve decisions at dark signals without resolving communications failures, emergency-route prioritization, curbside parking constraints or the operational challenge of recovering hundreds of vehicles during a citywide emergency.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What happened during the July 2026 outage?
A smaller PG&E outage on July 18, 2026 provided a limited follow-up. Reporting by TechCrunch and the San Francisco Chronicle said Waymo paused San Francisco service for about an hour and later resumed it.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →That quicker pause suggests a more conservative operating response after December. It does not establish that the underlying risks have disappeared, nor does it prove that the December software and emergency-protocol changes fully resolved the issue. It does show that the company’s response policy changed in at least one later outage.
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What regulators are examining
The CPUC’s proceedings have focused on questions that extend beyond one blackout:
- Why did vehicles stop in traffic lanes or intersections?
- How did Remote Assistance function when requests surged?
- Could first responders reliably contact Waymo and obtain rapid help?
- What support was available to passengers?
- Should autonomous passenger services face additional safety and reliability requirements for infrastructure failures?
The broader policy question is not merely whether an autonomous vehicle can drive without a working traffic light. It is whether a commercial fleet can fail gracefully during a citywide emergency—by recognizing degraded conditions, limiting new trips, preserving emergency access and clearing its own vehicles quickly.
What remains unanswered?
The public record still leaves important accountability questions open:
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- How many unique vehicles stopped, as opposed to how many stoppage events were recorded?
- How many blocked emergency routes, and for how long?
- What was the longest individual stoppage?
- What staffing and capacity were available for Remote Assistance during the peak?
- Why was service not paused earlier, before conditions became so difficult?
- Were first-responder contact systems tested at the scale required by a major outage?
- What independent validation has been conducted since the announced software update?
Answering those questions would make it possible to distinguish a one-time surge-management failure from a more persistent weakness in autonomous fleet emergency planning.
The larger lesson for robotaxis
The December outage exposed a problem that is easy to miss when autonomous driving is evaluated one vehicle at a time. A robotaxi may make a cautious decision that is locally defensible, yet hundreds of similar decisions can create a citywide operational problem when infrastructure fails.
Future emergency-readiness standards may need to address:
- real-time fleet-location access for city emergency managers;
- dedicated first-responder hotlines with guaranteed response times;
- automatic service restrictions when traffic signals or communications systems fail;
- clear authority to order an immediate fleet shutdown;
- procedures for moving or recovering vehicles that stop in emergency routes;
- public reporting of vehicle locations, stoppage durations and recovery performance after major incidents.
Human drivers are not automatically safer in a blackout. They can misread intersections, ignore improvised traffic control or block emergency vehicles. But an autonomous operator has a different responsibility: it controls a coordinated commercial fleet and can potentially detect a citywide pattern before individual vehicles become obstacles.
That is the lasting significance of San Francisco’s outage. The event was fundamentally a power and traffic-signal disaster, not a robotaxi-created blackout. Yet Waymo’s vehicles demonstrated that safe individual fallback behavior is not enough. The fleet also needs scalable remote support, fast emergency coordination and a proven way to get out of the road when the city’s infrastructure fails.
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