The question was: California motorists dread "Sig Alerts." Who or what is a "Sig?"
The first person with the correct answer was William. If William can send me his full name, shirt size and address by e-mail (firstname.lastname@example.org) I can send him his T-shirt.
A Sig Alert is defined by the California Highway Patrol as "any traffic incident that will tie up two or more lanes of a freeway for two or more hours, as opposed to a planned event like road construction."
Sig Alerts are issued by the CHP and are posted on their Web site, broadcast on radio and television stations throughout California
According to Wikipedia, Sig Alerts got started in 1955 because Los Angeles was having an increasingly bad problem with traffic jams and accidents. The police didn't want to call radio stations with information about the jams, so each station would call the police, jamming up the phones.
In 1955, Loyd C. "Sig" Sigmon, a broadcaster, developed a specialized radio receiver that would pick up traffic problems from the police. The LAPD refused to cooperate unless the receivers were made available to all radio stations. The "Sig Alert" system is now used throughout California.