I find it's a good idea in these situations (where you can really FEEL the person is going to do something stupid or not notice you) to pulse or flash the horn a few times well in advance. If you're not constant blowing the horn and just pulsing it, they 1)notice you, 2) realize they wouldn't have seen you otherwise and so understand what you're doing. I do it for these left turners and also blind turns, hills and so on.
