Question:  People were flashing their headlights at me all day yesterday.  Why?  

Answer:  You may have been driving in the rain without your headlights on.  California has a somewhat new law (passed in July 2005) that requires drivers to have their headlights on when weather is wet enough for use of your wipers.  The justification is simple, a moving car is easier to see when its headlights are on.  In wet, foggy, or misty weather, visibility is less than it is on a clear day.  Compensate for that by putting your headlights on.  Help other people instantly know your car is out there and keep us all safer.  See http://www.dmv.ca.gov/pubs/vctop/d12/vc24400.htm for the current text of the law.