Two cars flipped on the 405 Freeway during Thursday morning rush hour, triggering a massive backup across the Westside.
The collision happened shortly before 7:30 a.m. On the northbound side near Santa Monica Boulevard, according to the California Highway Patrol.
At least one person involved in the crash declined to be taken to a hospital.
Authorities had not immediately said whether anyone suffered serious injuries.
The wreck left the overturned vehicles blocking the freeway’s slow lane.
Emergency crews rushed to the scene as traffic quickly stacked up behind the crash.
CHP issued a SigAlert shortly after 8 a.m. For closures affecting lanes 4, 5 and 6.
The alert was later extended by another 30 minutes as crews worked to clear the wreckage.
The 405 Freeway is one of the busiest and most congested highways in the United States, carrying hundreds of thousands of vehicles daily through the Los Angeles area.
SigAlerts are issued by the California Highway Patrol to warn motorists of unplanned events that block traffic lanes for at least 30 minutes.
Thursday morning’s incident added to the corridor’s notorious reputation for rush-hour gridlock.
The northbound stretch near Santa Monica Boulevard is a particularly heavy traffic zone where multiple on-ramps and off-ramps converge.
Cleanup efforts continued past the initial estimated time as responders worked to right the overturned vehicles and clear debris from the roadway.
Drivers were advised to seek alternate routes through the Westside as the backup extended for miles.
The California Highway Patrol is investigating the cause of the collision.
No further details about the vehicles or occupants were immediately released by authorities.
Traffic conditions on the 405 are monitored continuously during peak commute hours due to the high volume of vehicles traveling between the San Fernando Valley and Los Angeles International Airport corridor.
Morning rush on this stretch typically peaks between 7:00 a.m. And 10:00 a.m., making incidents during this window particularly disruptive for regional mobility.
Caltrans and CHP coordinate responses to clear incidents rapidly, though multi-vehicle rollovers require extensive resources to address safely.
The extended SigAlert reflected the complexity of removing two overturned vehicles from active freeway lanes while maintaining responder safety.
Motorists caught in Thursday’s backup faced significant delays reaching destinations across the Westside and surrounding areas.