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ROSEMEAD, Los Angeles County, Calif., Sept. 21, 2026 — A four-vehicle crash blocked the express lanes and the number one lane of eastbound Interstate 10 near Walnut Grove Avenue Sunday afternoon, September 20, according to the California Highway Patrol. The agency's log describes a black SUV colliding with three other vehicles.
The California Highway Patrol's initial reports described a gray SUV, a white sedan and a brown Scion sedan involved in the crash, with radio traffic later noting white sedans and two other vehicles tangled in the express lanes. Callers reported adults and children standing in the traffic lanes after the collision, and one vehicle was reported smoking. Fire crews rolled a precautionary engine in response, and the Los Angeles County Fire Department was notified under its own incident number.
Some injuries were reported, though their number and severity have not been released. The crash blocked the middle lanes and express lanes as well as the number one lane, backing up eastbound traffic near Walnut Grove Avenue. The California Highway Patrol requested two tow trucks to clear the scene, and towing companies were dispatched with estimated arrival times of about 20 minutes.
The cause of the crash has not been determined and remains under investigation. We wish those injured a swift and full recovery.
Why Multi-Vehicle Pileups Often Snowball on the Freeway
A pileup rarely starts as a four-car event. It usually begins with one vehicle slowing or stopping unexpectedly, often in a high-speed lane like an express or HOV lane where drivers don't expect sudden braking. The vehicles behind have only a second or two to react, and if following distances are tight, the first impact triggers a chain of secondary collisions as each following driver runs out of room to stop.
Express and carpool lanes can make this worse because they're separated from general traffic and sometimes narrower, leaving less space to steer around trouble. Once one vehicle is disabled in the lane, drivers approaching at highway speed may not see it until they're already on top of it, especially if visibility is cut by glare, traffic density, or a curve in the roadway.
According to the National Highway Traffic Safety Administration, 365 large-truck occupants were killed in multi-vehicle crashes involving large trucks in 2023. That figure reflects how quickly one collision can multiply into several once traffic is moving at speed with vehicles bunched close together.
Frequently Asked Questions
How can drivers avoid getting caught in a pileup?
Keep a consistent following distance, watch brake lights several cars ahead instead of just the car directly in front, and be ready to slow gradually rather than braking hard at the last moment.
What should I do if I'm stopped near a crash on the highway?
Turn on hazard lights, stay in the vehicle with a seatbelt on if it's safe to do so, and move to the shoulder only if the vehicle can be driven safely. Standing in traffic lanes, even after a crash, puts people at serious risk from other vehicles.
How does fog or low visibility contribute to pileups?
Reduced visibility shortens the time drivers have to see and react to slowed or stopped traffic ahead, which is why chain-reaction crashes often cluster in fog, heavy rain, or glare conditions.
Why does following distance matter so much?
The gap between vehicles is the only buffer a driver has to stop or steer away from trouble. A shorter gap means less time to react, which is often the difference between a near miss and a collision.
What to Do After a Multi-Vehicle Crash in Rosemead, CA
If you're involved in a multi-vehicle crash, check on yourself and any passengers before doing anything else, and call 911 if there's any doubt about injuries. Move to a safe area away from moving traffic if you're able to, and avoid standing in open lanes even briefly.
Seek a medical evaluation even if injuries seem minor at first. Adrenaline can mask pain from soft-tissue injuries or concussions that show up hours or days later. Get contact information from other drivers and witnesses, and request a copy of the official collision report once it's available, since it can be useful for insurance purposes.
