Depends upon where you are. I live in the far suburbs verging on rural. Using a turn signal here gets you a dumbfounded look, and traffic waiting to turn out will just sit there. They don't believe you.
On my commute through parts of three congested cities or towns, the attitude changes, really as soon as multilane surface roads appear. There are the people, mostly women, who regard a turn signal as a magic forcefield that causes traffic to part for them like the Red Sea or something. They start veering over then flip on the signal while blabbing on the phone and glancing in the mirror. They're coming over, by the way. Might as well brake.
Then there are the hotheads, mostly male, territorial, hypercompetitive and just general a-holes who speed up to prevent you from changing lanes, I've had drivers intentionally speed up to keep me from merging, then brake hard to still keep me from even falling in behind.
As the population increases, the drivers seem to get worn down and less prone to aggression after a while. At first they're losing it because everything takes so much longer but then they just give up and ease off.
My expierence comes from being stationed at Camp Lejune. I saw it all over eastern NC.