To a certain degree, wearing sunglasses in a heavy rain can help you increase your visibility. I know you will think it is strange and at first I think so, however, now I believe it, because I have experienced the fact. As you know, when we are in the rain, the sunglasses will ease the distraction of raindrops hitting your eyes then glare is not much an issue during a rainy day, so we can see clearly in the rain (including the heavy rain). I am a girl with glasses; nevertheless, I would like to wear my sunglasses in rainy day, which can help me enjoy the light and clearness of the world. Of course, there is some trouble if you do so, for example, you will have to clear up your lens from time to time as rain will build up on the lens, and visibility will be reduced.
No, it is not true. The purpose of sunglasses is to help your eyes prevent from the uv and other strong sunlight. They are usually worn under the strong light, such as the beach, seaside and so on. However, it is polarized glasses that are more suitable for the drivers than the sunglasses, even if in a heavy rain whether.
Yes, it is true. Wearing sunglasses in a heavy rain can help increase visibility. That is right! The first reason of it is, the sunglasses would filter extra rays from the rain water and rain drops on your car window, that will keep your attention concentrate. The second reason is, if you wear a pair of polarized lenses sunglasses, it would filter parts of scattered lights which vertical to your eyes, the objects will be clearer. However, please do not wear dark colored sunglasses, because it will be dark in the cloudy day, it is too danger to drive in a dark vision.