Is the 'slow hand signal' for motorists or other cyclists?
I really don't think any motorist is going to follow my cue to slow down, they're going to do what they want and I'm getting out of the way as fast as I can. For other cyclists that I'm riding with, I'm always pointing at upcomming obstacles in the road as we approach them, or pointing with a finger which way we have to turn. I never use the right angle elbow bend signals unless my junker car is acting up again.
Use a mirror? .... nope.
jw