I've gotten so I just grit my teeth and accept the "courtesy". Folks just don't understand that if they don't slow and cause me to wonder whether they are actually going to stop for me, we'd all get through on our selected paths much faster. I do try to be nice and when someone stops to let me out, but the opposite traffic is heavy, I'll just wave them to continue. They do seem to get miffed when I don't accept their invitation to ride into danger. I've only had a few nasty ones in the last 10 years - motorists who slow down significantly and wave you on only to accelerate when you commit.