SigAlerts can be a harbinger of stress when you’re driving or using public transportation. They warn you of traffic jams. But did you know the alert system actually started as a way to share civil defense updates? We look into how SigAlerts began, and the man behind them.

What is a SigAlert? It’s any unplanned event that causes the closing of at least one lane of traffic for 30 minutes or more.

How did it start? It was created in the early ‘50s by Loyd Sigmon, a radio executive at L.A’s KMPC station. During the Cold War, civil defense officials were looking into how to make an alert system, a project that Sigmon developed.

So how did it get into traffic? It started out as a civil defense mass communication tool, but Sigmon also found a daily use for it: traffic alerts. He figured out how to make a special receiver that the LAPD could use to share a wide range of traffic updates.

There’s a term you’ve probably come across more than once while traveling around Southern California: SigAlert.

It’s one of the best ways to keep on top of traffic delays in near real-time, but they’re also a harbinger of stress. If a SigAlert is issued on your route, good luck getting to your destination on time. There could be a car stalled on the freeway, a car crash or any number of other problems shutting down lanes.

But do you know why they’re called SigAlerts? They actually started out as a radio tool.

How did SigAlerts begin?

Before SigAlerts lived online in an interactive map, and were issued by the California Highway Patrol, they came from a specialized radio receiver that broadcast recorded messages.

In the early ‘50s, civil defense officials had been looking into how to make an alert system in case of an attack from the U.S.S.R., which became the brainchild of Loyd Sigmon, executive vice president at L.A.’s KMPC radio station.

What defines a SigAlert?

  • It’s any unplanned event that causes the closing of at least one lane of traffic for 30 minutes or more, according to its website.

The system was first officially tested in November 1954. In City Hall, officials pressed a red button that took KMPC off air and instead broadcast civil defense information to listeners. Think of it like a Cold War version of the emergency alert system — the idea was to get urgent updates out fast, like potential attacks or major flooding.

A number of Southern California broadcasters signed on to install the necessary equipment, including KNX. Devices were also installed at the offices of civil defense personnel to speed up the process.

Where to find SigAlerts

  • Yes, you can look up SigAlerts and other real-time traffic information yourself by visiting

Sigmon also thought of a more daily use for the system. He approached the L.A. Police Department with an idea: set up a process for police departments to call the station when major traffic jams occur. But that wasn’t really feasible for officers to do every time.

It’s unclear how many times Sigmon customized devices to use the SigAlert system in different ways, but according to SigAlert researcher Harry Marnell, for the LAPD, he used a $600 shortwave receiver and tape-recording device that booted up with a special tone. The LAPD would receive details from other agencies and then press a button to record and send that special tone with the information to radio stations (like a high-stakes telephone game).

The system, which was put into widespread use in Greater L.A. on Labor Day 1955, was intended to tell the public about a range of concerns: major freeway tie-ups, smog alerts, fire, explosions, dense fog and “atomic” attacks.

Today, it’s just about major traffic delays. Relaying of emergency information lies with things like the emergency alert system. But Sigmon has gone down in history as pioneering a way for mass communication. He died in 2004 when he was 95.

How an early SigAlert caused traffic

Ironically, a SigAlert sort of created a big traffic jam.

Did you know?

  • Before Loyd Sigmon died, he was known to drive around in a cream-colored Lincoln Continental coupe with the vanity license plate “SIGALRT.”

  • The Santa Monica Pier’s bumper cars are also named after Sigmon’s work, dubbed Sig Alert EV. Their website says “Our Sig Alert EV is a great way to blow off steam after an hours-long commute to the west side”

In January 1956, a major train derailment happened — the Santa Fe train wreck — and an alert went out asking for any available doctors and nurses to respond. But so many people responded — including nosy onlookers — that it turned into an even bigger mess.

Priests, who were asked to come for spiritual help, were caught in traffic or turned away because of the amount of people driving over.

The derailment killed 30 people and injured 117. But the LAPD’s use of the SigAlert called into question exactly how effective (or ineffective) it was to broadcast such requests for help without a way to control the traffic build-up.

The L.A. Times put out an editorial days after the wreck that called out local authorities for poorly managing the emergency.

And while it praised how the SigAlert brought a swift medical response, the editorial board wrote that “nobody had the wit to stop” the onlookers on the highways.

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