What does it mean when someone's turn signal is blinking?
It means that it's working. You can't assume that they're going to turn. Either people never use them or the drive 5 miles with the blinker still on.
Apparently it is much too difficult for people to flick the indicator stalk before they turn. It drives me nuts. It's not even like they have to unclick it since. Jeesh! I always signal unless there is absolutely no one around. I do like the newer BMW feature that blinks the indicator 3 times with a push on the stalk.
Whether the person signals or not, I really hate how people seem to not understand that the round thing they're holding is meant to be turned when they are turning a corner and when they turn right they swing into the far left lane, or when they turn left the turn into the far right lane. Here is California it's illegal to do that but it's rarely enforced. Most drivers are just lazy and don't understand that driving takes a little effort sometimes. Common occurence: two lanes turn left and the inside car swings wide to the right and hits/nearly hits the car turning in the outside lane.

That's why in those situations I try to stay in the inside lane.