SAN MATEO, San Mateo County, Calif., Sept. 22, 2026 — A five-vehicle crash blocked lanes on northbound US-101 near the Peninsula Avenue on-ramp on Monday evening, September 21. At least two people were reported hurt.
The California Highway Patrol says the crash was first reported as a possible pickup truck and SUV colliding with another vehicle, before radio traffic indicated four vehicles were involved. Within minutes, dispatch logs show the count had grown to five vehicles, blocking all lanes on the northbound side near the on-ramp.
A San Mateo County ambulance was requested for injuries, and one report described a driver reaching a vehicle's built-in emergency assistance system after a crash that left one car unable to drive. Another vehicle involved was described as having non-life-threatening injuries but remained drivable enough to be moved to the roadway's right shoulder, per the log. CHP initially classified the incident as an unknown-injury collision before updating it to a minor-injury crash.
Responders worked to clear the scene by shifting vehicles first to the right shoulder and later to the left, gradually reopening lanes that had been fully blocked. Two tow companies were requested to remove vehicles described as suffering major front-end and side damage. The extent of injuries and how the chain-reaction crash began have not been detailed further by investigators.
We hope those who were hurt recover fully and quickly.
What Actually Reduces Chain-Reaction Freeway Pileups
Multi-vehicle pileups like this one usually start with one abrupt slowdown that ripples backward through traffic faster than following drivers can react. In heavy freeway flow, the gap between cars is frequently too short for the driver several vehicles back to see the initial brake lights, process them, and stop in time, especially where an on-ramp merges and speeds vary widely from car to car.
Engineering changes have been shown to ease that problem. Variable speed limit signs and ramp metering smooth out the speed differences that lead to sudden braking. Better lighting near merge points and clearer lane striping give drivers more warning before they reach a slowdown. Some corridors have added sensors that detect stopped traffic and trigger warning signs upstream, buying following drivers a few extra seconds to react.
Driver spacing still matters as much as any of that. Chain-reaction crashes concentrate at merge points and lane drops precisely because that's where speed changes are sharpest and least predictable, so following distance and attentiveness there carry more weight than they do on an open stretch of highway.
According to the National Highway Traffic Safety Administration, 28,595 large-truck occupants were injured in multi-vehicle crashes involving large trucks in 2023.
Frequently Asked Questions
Why do chain-reaction crashes happen?
They typically start with one vehicle braking hard or stopping unexpectedly. Following drivers who don't have enough space or time to react strike the vehicle ahead, and the collision spreads backward through traffic.
How can drivers avoid being caught in a pile-up?
Keeping extra distance from the vehicle ahead, especially near merge points and ramps, gives more time to slow down safely if traffic suddenly stops.
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 not safe to exit, and move to the shoulder only if the car is drivable and it's safe to do so. Watch for approaching traffic before stepping out.
How does fog or low visibility contribute to pile-ups?
Reduced visibility shortens the distance drivers have to spot a slowdown ahead, which is why multi-vehicle crashes cluster in fog, heavy rain, or after dark.
What to Do After a Multi-Vehicle Crash in San Mateo, CA
If you're involved in a multi-car crash on the freeway, try to move to the shoulder if your vehicle is drivable and it's safe to do so, and turn on hazard lights right away. Check on passengers and nearby drivers, and call 911 if anyone is hurt or vehicles are blocking traffic.
Even if injuries seem minor at first, it's worth getting checked by a medical provider, since some injuries from sudden stops don't show symptoms right away. Get a copy of the CHP incident report once it's available, and keep notes on what you remember while it's fresh, including lane positions and how the collision unfolded.
