Same here! The answer seems to be when rain or other precipitation is actually falling - see the bottom of this thread: https://www.honestjohn.co.uk/forum/post/index.htm?t=24691
Well it's all about statistics isn't it. Some people do lose concentration. Some people have to drive after having had 3 hours sleep in five days due to young kids and working all the hours in the day to make ends meet. Some people have a cold. Some people have a momentary lapse.
They've recently shown that 50mph limits through roadworks can increase the danger to workers as people lose concentration going so slowly on a motorway .