Rain and traffic.
OK, yeah, I agree, when it rains you should drive slower. Especially when there is a lot of traffic. You have to be careful and take notice of others.
But there is a limit to the speed drop to which one should be held. If one goes too slow then that car is blocking the way for others. Also abrupt change of speed by braking is just not done. The traffic behind these drivers congests until the point where I drive comes to a complete stand still.
This always happens and most annoyingly right after summer holidays when all the drivers with low intelligence return from holiday and start criss crossing all over the road changing lanes and passing on the wrong side.
There should be a law against it.
At least one.