Turn on headlights
EDITOR: Now that it's raining, I'm reminded that a surprising number of drivers don't seem to grasp the point of turning on their headlights when visibility is greatly reduced by water, mist, low light and general lack of contrast and definition — what skiers refer to as "flat light."
I guess those drivers are operating under the mistaken idea that they don't need their headlights on because they can see well enough, but that misses the point. It isn't about you not needing your headlights to see; it's about other drivers being able to see you.
Even though lights-out drivers benefit from the fact that most other drivers have their headlights on in the rain, making them visible at a considerable distance, they don't think to give other drivers the same courtesy and advantage of seeing them, as well, and this greatly increases the chances of an accident. The risk is heightened because most drivers are tuned in to watch for oncoming headlights while scanning traffic flow in daytime rainy conditions.
If this common-sense rationale isn't enough for you, it's also state law: If your windshield wipers are on, you must have your headlights on. It isn't optional.