Periodically wipe the lenses with your finger to get an even film of water, not droplets. (In cooler weather, consider wearing gloves that wipe well or that pull off easily so you can use your finger. Visors can help a lot in some rains for a while. In the daytime, Polaroid sunglasses often work well.

I've been battling this challenge for 55 year. Some rides are simply losers and I have to slow to safer speeds but I usually get the finger wipes close to right. Some rains require repeats often. I've thought about wipers many times but haven't put the inventor's cap on yet.