Big rig cargo fire shut down most southbound Highway 101 lanes near Portola, snarling traffic Wednesday

What happened

A big rig’s cargo caught fire Wednesday morning on southbound U.S. Highway 101 in San Francisco, triggering an extensive traffic backup and lane closures near the Portola neighborhood. The incident was reported at about 10:45 a.m. in the stretch south of the Interstate 280 interchange, an area where several major connectors funnel commute traffic toward the Peninsula.

Emergency response and traffic control operations led to the closure of most southbound lanes. The far-left fast lane remained open as crews worked at the scene. In addition, the southbound off-ramp to Third Street was shut down for a period during the response.

Traffic impacts and closures

The lane reductions quickly created a backup that extended for miles, with congestion building north of the I-280 connection. With limited southbound capacity, traffic conditions deteriorated not only on Highway 101 but also on nearby approaches feeding into the corridor.

• Most southbound lanes were blocked, leaving only the fast lane open.
• The Third Street off-ramp was closed during the incident response.
• Traffic backed up for miles north of the I-280/101 area.

Response and investigation status

Authorities managed the immediate hazards associated with the fire and maintained traffic control around the affected lanes and ramp. As of late Wednesday morning, officials had not released additional details about what caused the cargo to ignite, whether the tractor or trailer sustained major damage, or how long the remaining closures were expected to last.

There was also no confirmed public information by late morning regarding injuries, the type of cargo involved, or whether any secondary vehicles were affected. Investigators typically review driver statements, vehicle condition, and roadway factors after such incidents, while fire personnel assess burn patterns and potential mechanical or load-related sources of ignition.

Drivers encountered major slowdowns on southbound Highway 101 after a big rig’s cargo caught fire near the I-280 area, leaving only one lane open.

What commuters should know

With the scene located near a critical junction of San Francisco freeways and surface-street exits, delays can persist even after flames are extinguished, due to towing, cleanup, and inspections for roadway damage. Motorists traveling southbound through San Francisco were advised to anticipate ongoing congestion, monitor official traffic updates, and consider alternate routes until all lanes and ramps fully reopened.