I think the best solution for GPS on a bike is probably Google Maps on some sort of phone receiving input from it only through TTS over headphones in your helmet. If you needed to interact with it you would just pull over first. Everything needed for that solution probably exists already.

This is already how I do navigation while in my car, though in a few (increasingly rare) cases I still find myself looking at the screen for clarification. Iron out those cases and screen-less will become the best way to use navigation.