SAN MATEO, San Mateo County, CA, Sept. 25, 2026 — Four vehicles collided Thursday afternoon, September 24, on southbound US-101 near the Hillsdale Boulevard off-ramp, according to the California Highway Patrol. At least one person was hurt, the agency reported.
The crash was first reported shortly before 4:20 p.m. as three vehicles blocking the right-hand lanes, according to CHP dispatch logs. Within minutes, officers confirmed a fourth vehicle was involved and requested paramedics for a person in a Toyota Corolla who reported injuries after the vehicle's airbags deployed.
The CHP said all involved vehicles were being moved to the highway shoulder as officers worked to clear the scene. One vehicle, described in dispatch records as a Toyota Corolla with front-end damage, was towed from the scene. Traffic backups were expected in the area during the response.
The circumstances that led to the pileup have not been released. The extent of the reported injuries also remains unknown, and it is not clear how many of the four vehicles sustained damage beyond the Corolla. The crash remains under investigation.
We hope those who were hurt recover fully and quickly.
How Misconceptions Impact the Outcome of Multi-Vehicle Pile-Ups
Many drivers assume chain-reaction crashes happen mainly because of reckless driving. In reality, most pile-ups start with much less: a sudden slowdown, a lane merge, or one driver braking hard enough that the vehicles behind cannot react in time.
According to the National Highway Traffic Safety Administration, 28,595 large-truck occupants were injured in multi-vehicle crashes involving large trucks in 2023. That figure reflects how crashes involving more than two vehicles produce injuries, not just property damage, even when no single driver was speeding or impaired.
The bigger misconception may be that these crashes are unpredictable. Traffic data consistently shows they cluster at predictable points, off-ramps, merge zones, and spots where traffic slows unexpectedly, which is exactly the kind of location involved in this crash.
Frequently Asked Questions
What causes multi-vehicle pile-ups?
They typically start with one vehicle slowing or stopping suddenly, often near an off-ramp or merge point, followed by a chain of drivers unable to stop in time behind it.
Why do chain-reaction crashes happen so quickly?
Following distances shrink in heavy traffic, so when the lead vehicle brakes, each driver behind has less time and space to react, and the effect compounds down the line.
How can drivers avoid getting caught in one?
Leaving extra space behind the vehicle ahead, watching brake lights several cars up, and slowing early near ramps and merge points can reduce the chance of being pulled into a chain-reaction crash.
What should I do if I'm stopped near a crash on the highway?
Turn on hazard lights, stay in the vehicle if it's safe to do so, and move to the shoulder only if the vehicle is drivable and it's safe to merge.
What to Do After a Multi-Vehicle Crash in San Mateo, CA
Anyone involved in a multi-vehicle crash should first check for injuries and call 911 if anyone needs medical attention, even if the injuries seem minor. Airbag deployments and sudden stops can cause injuries that aren't obvious right away, so a medical evaluation is worth getting even for those who feel fine at the scene.
If it's safe, move vehicles out of traffic lanes and turn on hazard lights while waiting for officers to arrive. Exchange information with other drivers and take photos of vehicle positions and damage before any towing takes place. Filing a report with the California Highway Patrol and keeping a copy for insurance purposes can help sort out what happened once the investigation is complete.
