My wife has that issue, luckily she is on the back and just slaps me6. HAND SIGNALS
This one gets used a lot when I go riding with my friends. My girl-friend has a badder the size of a thimble and has to go every 10 minutes. We use the simple "thumbs down" sign for her to indicate it's time to pull over again. I even pack Kleenex on rides now for when the trees are the only shelter she may have.