What is a "sig" alert?

A sigalert is any sort of unexpected event that may interfere with traffic for a half hour or more, like a traffic collision, or a stalled car blocking lanes, etc.

Wiki says: A Sig Alert is defined by the California Highway Patrol as “any unplanned event that causes the closing of one lane of traffic for 30 minutes or more, as opposed to a planned event like road construction, which is planned separately.”