I'll give you one guess.
Poor weather conditions make it more likely that a car crash will occur. In fact, one quarter of US car accidents include weather conditions as a factor, according tot he Federal Highway Administration. Particularly hazardous are rainy and wet roads. Three out of four weather-related car crashes occur on wet pavement, and almost half happen during rainfall. It's interesting to note that only 12% of weather-related crashes occur on icy roads, and only 14% on snow or slush-covered roads. Yet one would think that icy roads are far more dangerous than wet ones. The reason? Drivers are more cautious on icy roads than on mere rainy roads. So they slow down when it snows. When it rains? Not so much.
Think of the factors that affect your drive when it rains. Tire traction decreases. This increases slipping and extends brake-time. Wet breaks can fail. Visibility is decreased. And hydroplaning can occur in a split second. Read more about hydroplaning dangers and what to do if your car hydroplanes on this previous blog post.
It stands to reason that if rain-related accidents occur at five times the frequency of snow-related crashes, our rainy state of Washington's laws should reflect that. Sure enough, Washington has some unique laws when it comes to rain and driving laws.
Here's Washington State Law RCW 46.37.020 on use of headlights:
Every vehicle upon a highway within this state at any time from a half hour after sunset to a half hour before sunrise and at any other time when, due to insufficient light or unfavorable atmospheric conditions, persons and vehicles on the highway are not clearly discernible at a distance of one thousand feet ahead shall display lighted headlights, other lights, and illuminating devices as hereinafter respectively required for different classes of vehicles, subject to exceptions with respect to parked vehicles, and such stop lights, turn signals, and other signaling devices shall be lighted as prescribed for the use of such devices.
Therefore, a police officer has discretion about whether or not a motorist broke the law by not turning on the headlights during wet road conditions. The common sense takeaway is simply to turn on your headlights when it's harder to see, i.e. during rainfall. In fact, it's safer to keep your headlights on any time you drive, since you may encounter conditions that will help other drivers see you even in broad daylight, such as on rural roads or two-lane highways.
There is some ambiguity in the law regarding speeding and road conditions, again leaving some discretion up to the police officer on the scene. Here's part of the law that pertains to wet road conditions:
The driver of every vehicle shall, consistent with the requirements of subsection (1) of this section, drive at an appropriate reduced speed when approaching and crossing an intersection or railway grade crossing, when approaching and going around a curve, when approaching a hill crest, when traveling upon any narrow or winding roadway, and when special hazard exists with respect to pedestrians or other traffic or by reason of weather or highway conditions.
The officer on the scene has legal discretion to assess the factors and make a judgment call about how the driver could have acted safely but did not. Sometimes, even a posted speed limit will be too fast for the conditions. This is obvious on a mountain pass with a 70 mph limit but with blizzard conditions. Heavy rain is not that different. Police officers and lawmakers know that about 90% of car accidents include human error as a factor.