Speed limits exist to tell you how safe it is to drive under good conditions. When conditions are bad and roads are wet, speed limits are worthless. Drive well under them -- and the worse the conditions, the lower the speed you should drive.
The worst danger of driving too fast in rain is hydroplaning. No, hydroplaning isn't something you do when you're flying off on a fishing expedition in a Cessna that's equipped with pontoons. Hydroplaning is what happens when your car thinks it's a boat while it's still on the highway.
Usually your tires can slice their way through the water in front of them and keep in contact with the surface of the road. But when the road is wet and you're going too fast, your car can actually begin to float on top of the water and the tire tread loses contact with the road surface. This is bad. Boy, is this bad! When your tread loses contact with the road surface, you can no longer steer. You can no longer brake. This is what happens when you hydroplane. And you often don't know that you're hydroplaning until you hit the brakes and the car goes skidding out of control. Therefore it's better not to travel at hydroplane speeds to begin with.
What do you do if you realize you're hydroplaning and are already out of control? First off, don't panic (though, trust us, you'll be tempted to). Don't hit the brakes, because that just makes it worse. Let up on the accelerator so that any remaining traction can slow your speed. And drive straight. Don't try to turn. If the car is veering off in a direction you don't want to go, don't fight it; just follow your wheels. And as the car slows, suddenly (almost magically), you'll be back under control.
At this point we recommend getting off the road and giving yourself time for your heart rate to slow back down. You'll need it.