However being a member of the Soggy Bottom Riders Association, there are things that will minimize your obstacles of not seeing.
Keep your windscreened waxed....Dr. Scratch works wonderfully. You can find it at most Walgreens, Cvs Drugstores...thats right. Drugstore. It comes in an orange tube with a black cap. It lets the rain skim off the screen. It also removes scratches that start to accumulate on the windscreen from normal cleaning and wear and tear....
Next when you see that you are riding into inclimate weather, stop and put on a set of over the glass goggles, that have been treated with an antifogging agent, and also treat them with Dr. Scratch.... ON THE OUT SIDE ONLY>>>>may not work well with antifogging agent....I haven't tested it yet.
It doesn't eleviate all the problems with seeing however it makes it bearable. As a glass wearer myself, I have found that it allows me better focusing ability when my vision glasses are not being splattered with rain drops.
Then there just comes the time when the rain is so heavy the best thing to do is just find a place to pull over safely. It is hard to do at times.
But it makes for a great story about your Ad-VENTURE.
Have a great day....