Something should be done, statewide and nationwide, about enforcing the use of turn signals by licensed drivers!
The California Driver's Handbook says clearly that turn signals must be used when making a left/right turn or changing lanes, reasonably soon enough to let drivers behind you know of your intention.
I think that 80% to 90% of the drivers just don't know they have the turn signal-lever below their steering wheel! Drivers of both sexes and all ages!
It's aggravating and often hazardous, but all you can do is spit a curse or use your finger sign. It should be strongly enforced and ticketed by the CHP and police!