If you're driving through fog, heavy rain, a snow storm or on icy roads, be extra cautious. Take all of the other tips presented here and make full use of them: Drive below the speed limit if necessary, maintain extra space between you and the car ahead, and be especially careful around curves. If you're driving through weather conditions you don't know well, consider delegating driving duties to someone who does, if possible. If the weather worsens, just find a safe place to wait out the storm.
If you're experiencing bad visibility, either from fog or snow, and you end up off the side of the road (intentionally or otherwise), turn off your lights. Drivers who can't see the road will be looking for other cars to follow along the highway. When they see your lights, they'll drive toward you and may not realize you're not moving in time to avoid a collision.