I actually lift a hand in the extended stop position when it looks like the driver wants to turn left in front of me, and I know I'm going too fast for all of us to make it. (even followed our club bulldog once at 28 going uphill! and saw him use it on a car on the right hand side of the road.) cars are surprised when you do it, but I think the majority just don't know realize you're going that fast, and they do respect it.
I always nod and say thank you to cars who wait. even the ones I don't signal.

it i'm going under 18 mph, they guess how much time they have a little better and at most, I have to slow a few mph before they get out of the way.

none of this helps for the annoying people on the right side of the road, turning left, who pull out into my lane before even checking any direction for traffic... they pull out, stop in the lane, oh, I guess it looks clear except for that bike, then go... drivers like that are just asking to get hit...