Once I was driving on a highway, in the left lane, in heavy traffic, entering a tunnel at some 140kmh, when due to enormous temperature and humidity difference literally blanked out all my windows and windshield. The tunnel had no ventilation and was filled with fog as well. It was literally as if someone hit a switch that turned off my screen. I started braking, turned in emergency light and toggled windshield fan all in less than a second. You could hear other people's tires screeching around me as well. I was in many hard situations on road and in life, but this was blood chilling. I hate to think what would happen if I wasn't able to get my windshield clear again so fast. I drove blindly for 2-3 seconds in a tunnel at 140kmh mate. No pulling over, no stopping with hazards on since guy behind you probably can't see you either.