Yes. Those pesky bright reflections of the sun on the cars ahead can sometimes be attenuated a good deal. They tend to be horizontally polarized, thus perfect for vertically polarized sunglasses. The reason is that the surfaces that you see on the car in front of you (the back window, the trunk door, and even the roof) will be slanted towards you, while the sun will be more or less aligned in the vertical plane through both cars (if not you wouldn't see its reflection from those surfaces). However, if the sun is relatively low behind you, the sun rays will be near perpendicular to the reflecting (vertical) surfaces which won't polarize the light. The sunglasses will help more with the reflections from the glass than from the metal as the former are polarized to a higher degree. What about the brightness of the road itself? That light is also partially polarized, but by scattering (as opposed to reflection). The direction of polarization will change with the direction of the road with respect to the position of the sun. The rule is that the polarization is tangential to a circle centered on the sun. That means that if the sun is in front, behind, or high above, the road brightness will tend to be horizontally polarized and the filter of the sunglasses will provide some help. However, if the sun is to one side, the polarization will rotate vertically, the more so the lower the sun happens to be. Of course, if the road is wet you get the same anti-glare power than at sea.

Sometimes yes, sometimes no. The snow glare is not polarized, so they won't provide extra help in that regard (although often advertised for that purpose). But, with the sun high, the air-light (haze + sky) near the horizon is polarized by scattering and the polarized sunglasses can make features far away really stand out (this is used in fire detection). On the other hand, with the sun low the sunglasses could be detrimental looking south or north, as the air-light would be vertically polarized. Other situations can be thought were they would be useful, but remember that one reason they are so good in water sports is that the reflector always remains horizontal!