A serious multi-vehicle collision on U.S. Highway 101 in Petaluma, California, caused major traffic disruption on Saturday afternoon after authorities were forced to shut down all southbound lanes near the East Washington Street interchange.

According to the California Highway Patrol (CHP), the crash was reported shortly after 1:10 p.m. on southbound Highway 101 just north of East Washington Street. At least two vehicles were involved in the collision, though officials have not yet released full details on the number of occupants or the severity of injuries.

Emergency services, including CHP units, firefighters, and paramedics, responded quickly to the scene. First responders worked to secure the roadway, assess those involved, and begin initial medical evaluations. Witnesses reported a large emergency presence, with multiple sirens converging on the area within minutes of the incident being reported.

As crews worked at the scene, traffic along one of Sonoma County’s busiest transportation corridors came to a standstill. Southbound lanes were fully closed for a period of time, causing significant delays that extended for miles in and around Petaluma. Motorists were urged to avoid the area and seek alternate routes while cleanup and investigation efforts were underway.

Authorities established a perimeter around the crash site to allow emergency personnel to operate safely. Damaged vehicles were eventually removed from the roadway once preliminary assessments were completed, though full traffic restoration took several hours as congestion gradually cleared.

The closure had widespread impacts on weekend travel, affecting commuters, commercial drivers, and local residents. Nearby surface streets also experienced heavy congestion as drivers diverted away from the freeway shutdown.

Officials have not yet confirmed what caused the collision. Investigators are reviewing all possible contributing factors, including roadway conditions, driver behavior, and vehicle performance. At this time, no information has been released regarding speed, impairment, or mechanical failure, and the investigation remains ongoing.

The incident occurred along a key stretch of U.S. Route 101, a major north–south freeway that carries heavy daily traffic through Northern California. Crashes along this corridor often lead to significant regional delays due to limited alternative routes and high traffic volume.

Authorities have confirmed that evidence from the scene, including vehicle positions, roadway markings, and witness statements, will be analyzed as part of the ongoing investigation. Surveillance footage from nearby traffic cameras may also be reviewed to help reconstruct the sequence of events.

No official updates have yet been released regarding fatalities, though injury assessments were conducted at the scene. CHP officials stated that additional information will be provided once families are notified and investigative findings are confirmed.

As cleanup efforts concluded and lanes gradually reopened, normal traffic flow slowly resumed later in the afternoon. However, officials cautioned that residual delays and congestion could continue into the evening.

The California Highway Patrol continues to investigate the crash and is urging anyone who may have witnessed the collision to come forward with information that could assist in determining what occurred.

Further updates are expected as authorities complete their review and release additional verified details.