I actually have that turned off on purpose. Mainly for discussion threads as people are hesitant to reply to 'old' topics, even if they're relevant. The disadvantage is that comments on recurring event threads can end up seeming current, like what you experience.
I have seen those signs, and I bet they have caused accidents from people immediately slowing down and causing an accordion effect during rush hour. I've had to learn to be very careful driving the speed limit because there's not much you can do to avoid an accident from the rear.