SigAlerts Explained: The CHP Bulletin Behind the Traffic App Warning

Every Southern California driver has seen the word on a radio report or a map app. Fewer know what it actually means, and almost nobody thinks about the part of a SigAlert that hurts people: not the incident itself, but the mile of suddenly stopped traffic behind it.

What qualifies, according to the CHP

The CHP's dispatch manual divides traffic bulletins into two kinds:

The manual says a bulletin should give the exact location, direction and lanes affected, an approximate duration, and the cause, and that officers update the closure in real time as lanes reopen. So when you see a SigAlert, the useful information is the lanes and the expected duration, not the label.

The back of the queue is the dangerous part

A SigAlert turns a moving freeway into a parking lot without much warning. The collisions that follow come from drivers arriving at full speed behind traffic that stopped a minute ago.

• Watch further ahead than usual when an app or sign warns of a SigAlert, and start slowing at the first brake lights you see, not the last.
• Use your hazard lights briefly as you come up on stopped traffic, so the driver behind you sees the change in time.
• Leave room in front once stopped, so a hit from behind does not push you into the car ahead.
• Keep checking the mirror. If someone is arriving too fast, the room in front is your only escape.

Exit and detour, or sit it out?

If the bulletin says lanes will be closed for a long time and you are near an exit, getting off can make sense. Two cautions. Map apps send everyone to the same surface streets, which fill quickly and bring pedestrians, school zones and unfamiliar intersections into a drive that was a freeway trip. And cutting across lanes to reach an exit at the last moment is its own collision risk. If you will not make the exit cleanly, let it go.

Passing the incident itself

When you finally reach the scene, keep your eyes on the road. Slowing to look is how secondary collisions start, and responders, tow operators and stranded drivers are on foot nearby. Give stopped emergency vehicles a lane where you safely can, and slow down where you cannot.

How collisions in a SigAlert backup are judged

California's regulation on principal fault says a driver who is rear-ended and not convicted of a moving violation for the accident is presumed not principally at fault. In a chain-reaction collision in a sudden queue, that presumption is often the starting point for how each insurer reads the claim, though the facts decide it. Damage to your own car from being hit is the other driver's liability claim, or your own collision or uninsured motorist coverage if they have none; damage you cause to the car ahead is your liability coverage.

If your commute runs through the corridors that seem to produce a SigAlert every week, check that your collision and uninsured motorist coverage are what you would want on the day it is your lane.