If you've driven in Los Angeles, San Diego, Sacramento, or anywhere in California, you've heard it: "We've got a SigAlert on the 405 northbound at the 10 due to an overturned semi."
A SigAlert — also spelled Sig-Alert, Sigalert, or SIG Alert — is California's official term for a traffic incident that significantly disrupts road traffic. It's more than just a traffic report. It's a formal alert system, born in the 1950s, codified by the California Highway Patrol (CHP), and now a part of the English language. The New Shorter Oxford English Dictionary added it in 1993.
But what counts as a SigAlert? Who issues it? How did it get its name? And why does Los Angeles have a bumper-car ride called "SigAlert" at Pacific Park? This 2,500+ word article answers everything, with FAQs at the end.
I
Definition — What Exactly Is a SigAlert?
1.1 CHP Definition
The California Highway Patrol defines a SigAlert as "any unplanned event that causes the closing of one lane of traffic for 30 minutes or more".
In practice, the CHP issues a SigAlert notice, which is then:
- Posted on the CHP website and CHP Traffic Incident Page.
- Broadcast on radio and television stations throughout California.
- Signaled to motorists via electronic message signs on freeways.
1.2 Caltrans Definition
Caltrans (California Department of Transportation) uses a stricter standard: any traffic incident that will tie up two or more lanes of a freeway for two or more hours.
So CHP may issue a SigAlert for 1 lane/30 min, while Caltrans reserves the term for 2+ lanes/2+ hours. In daily use, media report both.
1.3 Spelling Wars
There is no standard spelling. Examples:
CHP: "SIG Alerts" / "Sigalert"
Caltrans: "Sig-Alert"
Wikipedia: "Sig alert" / "Sig-alert"
Oxford: Added in 1993
For this article we'll use SigAlert, the most common modern form.
II
History — Loyd Sigmon and the Birth of Real-Time Traffic
1955 — The Problem
Southern California's Growing Pain
Southern California freeways were already jamming in the 1950s. Police had no fast way to tell radio stations about accidents. Stations had no way to interrupt programming instantly.
1955 — The Invention
Loyd C. Sigmon's Breakthrough
Loyd C. Sigmon, a broadcast engineer and later executive at Golden West Broadcasters, invented a system in 1955 to send automated alerts. He created a special receiver placed at radio stations. When CHP had a major incident, they triggered a tone that opened the receiver, broadcasting the alert live.
He named it after himself: Sigmon Alert = SigAlert.
"Driving a cream-colored Lincoln Continental with a hands-free cellular telephone and a vanity license plate that proclaim, 'SIGALRT.'"
— New York Times, 1997, by Todd S. Purdum
1950s — Early Use
Entering LA Culture
The first SigAlerts were for collisions, jackknifed big rigs, and three-car pileups on the Santa Monica, San Diego, and Pacific Coast Highway. Radio DJs interrupted songs to read them. The term entered LA culture instantly.
1950s → 2020s — Evolution
From Radio to Digital
1950s
Radio
1980s
TV Crawls
1990s
CHP Website
2000s
Freeway Signs
2020s
Waze/Maps
III
How a SigAlert Works Today — 2026 Edition
3.1 CHP Process
1
Incident occurs
Crash, fire, spill, police activity.
2
CHP officer estimates closure time
Officer on scene estimates lane closure duration.
3
Threshold met → SigAlert issued
If ≥1 lane for ≥30 min, CHP dispatcher issues a SigAlert.
4
Alert includes full details
Location, nature of disruption, expected duration, lanes affected, recommended alternate routes.
5
Multi-channel dissemination
CHP website, media, Caltrans QuickMap, changeable message signs, third-party apps.
3.2 Sacramento Example
In Sacramento, CHP dispatchers verify the situation and broadcast to media and digital platforms. Alerts include: location, nature (accident, vehicle fire, debris), expected duration, and alternate routes. Navigation services like Google Maps and Waze integrate SigAlert Sacramento data to enhance routing.
3.3 San Diego Police Procedure
SDPD Policy 2.07 (Feb 27, 2019) governs Sigalert Bulletins. Key points:
Purpose: Advise public of emergency conditions.
Request: Any law enforcement agency; ranking officer must request, except CHP where any member can.
Dissemination: Local participating commercial radio stations.
So local PDs can trigger SigAlerts, not just CHP.
3.4 What Drivers Hear
"CHP has issued a SigAlert for the northbound 405 at the 10. Two right lanes blocked due to an overturned semi. Expect delays of at least one hour. Alternate: Use La Cienega."
Typical radio read format
IV
SigAlert vs. Other Traffic Terms
| Term | Meaning | Who Issues | Duration/Lanes |
|---|---|---|---|
| SigAlert | Unplanned event, 1 lane/30+ min (CHP) or 2 lanes/2+ hrs (Caltrans) | CHP, Caltrans, local PD | 30 min+ |
| Traffic Advisory | General delay, no lane closure threshold | Any agency | Varies |
| AMBER Alert | Child abduction | CHP | N/A |
| Quick Alert | Rail/police radiotelephony urgency | Railroads | N/A |
In radiotelephony, "SigAlert" is listed alongside "ten codes" and "Quick Alert!" as special phrases that abbreviate critical data.
V
Cultural Impact — From Freeways to Ferris Wheels
LA Culture
Todd Purdum, NYT 1997: "Each weekday morning in the City of Angels… the rat-atat-tat of traffic reports crackles… Then, when you least expect it, you hear the S-word."
Every Southern California driver knows it means "You're about to be delayed," says Layna Browdy of the Automobile Club of Southern California.
Pacific Park
When Pacific Park amusement arcade opened on Santa Monica Pier, it christened its blue-and-yellow bumper car ride "SigAlert." The term is that universal.
Language
Added to New Shorter Oxford English Dictionary in 1993. Used in TV, movies, and music to signal LA traffic hell.
VI
SigAlert in the Digital Age
Sigalert.com
Primary web destination
Provides "Los Angeles traffic reports. Real-time speeds, accidents, and traffic cameras." Check conditions on Ventura and Hollywood freeways, I-5, I-405. Email or text traffic alerts on personalized routes.
722.3K
Monthly Visits
4.69
Pages/Visit
#12,312
US Rank
2:15
Avg Duration
Similarweb data, March 2026
CHP Traffic Page
cad.chp.ca.gov
Official, real-time. No frills, but authoritative.
Navigation Apps
Waze, Google Maps, Apple Maps
Waze, Google Maps, Apple Maps ingest SigAlert data. When CHP issues one, your app reroutes you.
UCLA & Metro
Institutional use
UCLA Transportation lists Sigalert.com as a free service for real-time traffic. Metro offers its own real-time tool.
VII
SigAlert Outside California
While born in CA, the term appears elsewhere:
Jackson, MS
City sites run "Sigalert Sacramento" and "San Diego Sigalert" articles for SEO, but they're about CA.
Radiotelephony
"SigAlert" listed as special phrase in police/rail comms.
Azure Scam Alert
Attackers fake "Severity: 3 SigAlert-4255635-A16" emails to look official. Microsoft warns: real Azure alerts don't use phone numbers or "SigAlert" language.
If you see "SigAlert" outside CA traffic, be suspicious.
VIII
SigAlert Statistics and Thresholds
CHP
1+
Lanes
30+ min
Duration
Caltrans
2+
Lanes
2+ hours
Duration
Common Media
1+
Lanes
Any major
Delay
Most radio SigAlerts follow CHP standard. Caltrans reserves it for bigger events.
IX
How to Use SigAlerts as a Driver
Check before you go
CHP Traffic, Sigalert.com, Google Maps.
Sign up for alerts
Sigalert.com offers email/text for personalized routes.
Listen to radio
AM 1070 KNX, AM 980 KFWB in LA still read them.
Watch freeway signs
CMS will say "SIGALERT AHEAD."
Have alternates
Sepulveda for 405, La Cienega for 10, surface streets for PCH.
Bay Area tip
SigAlerts matter more because I-80, US-101, I-280, and Bay Bridge have few alternates. One incident ripples.
X
The Future — Will SigAlerts Survive?
Yes, but the name may fade. Waze and AI rerouting give alerts without saying "SigAlert." Still, CHP will issue them, and radio will read them, because "SigAlert on the 405" is faster than "Traffic incident 2 right lanes blocked 45 min."
Frequently Asked Questions
The S-Word That Runs LA
From Loyd Sigmon's 1955 radio hack to 2026 Waze push notifications, the SigAlert is California's way of saying "you're not getting there on time." It's technology, culture, and language all at once.
Next time you hear "SigAlert on the 10," you'll know: some lane, somewhere, has been closed 30 minutes, and Loyd Sigmon's Lincoln Continental would understand.