Fog (or poor visibility)

At certain times of year (chiefly autumn), moisture in the air will gather in the lowest parts of the road as the air cools in the evening.  If it's getting dark you may not see it until you're in it.  The problem's worse at night: you won't know you're driving through fog until light from your headlights is dazzling back at you from the millions of tiny droplets.

Optional low-mounted headlights ("front foglights") can throw enough light ahead of you to show how the road goes up or down without dazzling, because the light comes from below your eye level.

Compulsory rear foglights (bright red lights) make you visible to anyone behind.  Although they're called FOGlights, use them whenever you can't see at least 100m ahead (heavy rain, snow, dust).

Also reduce your speed, as following traffic may not be able to tell the difference between foglight and your brake signal.  (This is an advantage in having just one rear foglight, though two are common).

Watch the video to see how much easier it is to see cars, especially dark coloured ones, and assess their speeds through spray on a motorway.