We all know about the polarized sunglasses, and about the theoretical-full-darkness using Malus's law with two polarizers. We want to put it to use in the simplest way:
We add another layer to the polarized sunglasses that we use. This layer is a polarizer as well, and it is adjustable - it can be rotated around the axis normal to the lens; so that the user can choose what level of attenuation the glasses will supply. The user can rotate it so that both lens are parallel, for regular daywalk, or so that they are perpendicular in case he is driving against the sunset.