When friends came home, I let them try UbuntuTouch on my Nexus7. First, I just said "use edges".
They started and found easily the launcher, then launched gallery, then an image in full screen, and then... lost ! They were stuck on the image, tapping/swapping on the image as much/fast as they could.
I had to tell them "the toolbar is in the bottom edge and there is a back button".
All this made me think : "what if... there was an emergency mode ?"
Here is a proposal ;-)
If you have ideas about other gestures that can be mapped to the "emergency mode", please tell me !