What is the rule for using a car's emergency lights in the rain?

Question

I'm in Mid-South Carolina, and lately there have been several really heavy downpours to the point of you almost driving blind. I have noticed that many, but not all, motorists in these conditions turn on their flashing emergency lights. Is this a law, or are people needlessly warning you of the rain (it seems moot to have them on, we can see your lights anyways and we KNOW that its raining heavily)?

Answer 1

In a situation where you are "almost driving blind" another driver's use of the four way flashers may help you see that there is a car ahead of you. They are calling attention to their vehicle, and to the fact that they are going slower than normal. Any slow moving vehicle presents a hazard on the roadway.

"it seems moot to have them on, we can see your lights anyways " Flashing lights draw more attention than do non flashing ones. That is why emergency vehicles have flashing lights! The flashers also are brighter than the normal running lights.