I’d heard this term on the radio a lot on AM and I had no idea what it meant exactly. I asked a lof of friends, etc. and no one seemed to know. Finally today I read about a teenage girl jumping off a bridge onto the 101 in Palo Alto and it mentioned it again so I googled the term and found the official CalTrans definition:

“Sig-Alerts” are unique to Southern California. They came about in the 1940s when the L.A.P.D. got in the habit of alerting a local radio reporter, Loyd Sigmon, of bad car wrecks on city streets. These notifications became known as “Sig-Alerts.” Later Mr. Sigmon developed an electronic device that authorities could use to alert the media of disasters. Caltrans latched on to the term “Sig-Alert” and it has come to be known as any traffic incident that will tie up two or more lanes of a freeway for two or more hours.