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A major PG&E power outage on December 20, 2025 knocked out traffic signals across much of San Francisco. Some Waymo robotaxis stopped or became delayed at intersections, and Waymo temporarily suspended service while the city dealt with severe congestion.
But the evidence does not show that every Waymo vehicle was immobilized. Waymo says its autonomous Driver handled more than 7,000 dark signals during the outage. The main problem was the scale of the event: thousands of simultaneous signal failures generated a surge in requests for remote confirmation, creating delays as streets were already gridlocked.
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What happened during the San Francisco outage?
The outage began on Saturday, December 20, 2025, affecting approximately 130,000 San Francisco homes and businesses—nearly one-third of PG&E customers in the city. Traffic signals went dark across major corridors, public transit was disrupted, and officials urged people to avoid nonessential travel. Most affected customers had power restored by December 21, according to The Associated Press.
With intersections operating without signals, human drivers, pedestrians, police officers, transit vehicles and autonomous cars all had to navigate an abnormal traffic situation. Videos and eyewitness accounts showed multiple Waymos stopped or clustered at intersections. Some vehicles contributed to street blockages, although the available reporting does not establish how many Waymos were on the road or how many were affected.
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Waymo ultimately paused service in the affected area. It directed vehicles to pull over and park appropriately, then return to depots in waves as conditions allowed.
Why did Waymo cars stop?
The cars did not simply lose the ability to recognize a dead traffic light. Waymo says its Driver is designed to treat a dark signal as a four-way stop, and that its vehicles successfully crossed more than 7,000 dark signals during the outage.
The operational sequence was more complicated:
- The vehicle detected that a traffic signal was dark.
- It treated the intersection as a four-way stop.
- When conditions were uncertain, it could request a confirmation check through Waymo’s remote-assistance system.
- The widespread outage caused an unusually large number of vehicles to make such requests at the same time.
- The resulting backlog delayed responses for some vehicles.
Waymo described the issue in its account of the outage as a surge in confirmation requests, not as conventional remote driving of the entire fleet. The company’s explanation points to a capacity and coordination problem: a cautious behavior that may be manageable at one failed intersection became disruptive when repeated across a citywide network of disabled signals.
Was the entire Waymo fleet grounded?
No. “Grounded” accurately describes the temporary suspension of service, but it is too broad if it implies that every Waymo vehicle became immobile. The evidence supports a narrower conclusion: multiple vehicles were stopped or delayed, some contributed to congestion, and Waymo suspended operations while the emergency continued.
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Nor does the evidence show that the robotaxis’ onboard power, batteries or computers failed. The disruption was primarily caused by the loss of city infrastructure—especially traffic signals—and the resulting traffic-management emergency.
Why conservative driving became a traffic problem
Stopping is generally preferable to making an uncertain maneuver through a signal-less intersection. However, safety decisions made independently by many vehicles can have network-wide consequences.
- Safety versus flow: Waiting reduces the chance of a wrong move but can block lanes when many vehicles wait together.
- Vehicle awareness versus city awareness: A car can perceive its immediate intersection, but a citywide outage requires information about failed signals, police direction, blocked roads and emergency vehicles.
- Confirmation versus scalability: Human review can help resolve unusual situations, but a remote-assistance process must handle demand spikes.
- Continuing versus suspending service: Staying on the road preserves rides but may add congestion. Pausing service reduces new trips and allows fleet recovery, but removes transportation during an emergency.
The same distinction matters when comparing autonomous vehicles with human drivers. Human motorists also struggled with the blackout, congestion and manual traffic control. The autonomous-vehicle-specific issue was that a large fleet added a visible, repeatable failure mode to an already serious transportation disruption.
Were passengers trapped or injured?
Available authoritative reporting establishes delays, street obstruction and a temporary service suspension. It does not establish a broad pattern of passengers being trapped, abandoned or injured. Stopped-vehicle videos should not be treated as evidence of a quantified safety or casualty event.
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What did Waymo say it would change?
In a December 23, 2025 update, Waymo said it was rolling out changes to give vehicles more regional outage context, allowing them to navigate affected intersections more decisively. It also said it would improve emergency procedures and coordinate more closely with San Francisco officials.
Waymo further said it had trained more than 25,000 first responders in the United States and internationally and would update that training based on lessons from widespread disruptions. Those were announced responses; the source does not independently establish that every planned improvement had been fully deployed or validated by August 2026.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does the incident prove autonomous cars are unsafe?
No—but it does demonstrate a meaningful weakness in how an autonomous fleet handled an unusual infrastructure emergency.
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Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Waymo says it has accumulated more than 100 million fully autonomous miles and reports fewer serious-injury-or-worse crashes than human drivers in its operating areas. Those figures come from Waymo’s own analysis and should be understood as company-reported claims, with geographic and methodological limits. They address collision safety during the measured driving period, not necessarily traffic management during a mass outage. See Waymo’s safety analysis.
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A system can perform well on ordinary driving safety metrics while still needing improvement in four separate areas:
- Routine collision avoidance.
- Behavior during infrastructure failures.
- Fleet-wide traffic and emergency-response performance.
- Public confidence and social acceptability.
This is why a blackout affecting one dark signal is not equivalent to a blackout affecting thousands. The first is a familiar road condition. The second is a rare, city-scale systems failure involving utility infrastructure, traffic control, communications, emergency services and fleet logistics.
What regulators and cities should learn
The incident suggests that autonomous-vehicle oversight should evaluate more than whether an individual car can safely cross an intersection. Cities and operators also need plans for:
- Rapidly sharing outage and traffic-signal status with autonomous fleets.
- Scaling remote assistance during simultaneous failures.
- Giving emergency responders clear ways to identify and move autonomous vehicles.
- Keeping vehicles out of lanes, intersections and emergency routes during service suspension.
- Returning or repositioning vehicles when roads are blocked.
- Handling outages caused by earthquakes, wildfires, severe weather or cyberattacks.
- Communicating clearly with passengers and the public.
Future evaluations should also test edge cases such as police officers directing traffic, emergency vehicles approaching, poor visibility, cellular-network degradation and multiple outages occurring in succession. Different municipalities may use different emergency procedures, so a vehicle’s response cannot depend solely on a generic four-way-stop rule.
The bottom line
The December 2025 blackout did not show that Waymo cars were universally unable to handle dark traffic signals, nor that the entire fleet lost power or became permanently stranded. It showed something more specific and more important: when a large portion of a city’s infrastructure failed at once, Waymo’s confirmation and fleet-management processes did not scale smoothly. The company paused service and announced software, emergency-response and first-responder changes. Whether those changes fully solve the problem requires deployment and performance evidence beyond the initial announcement.
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