If you go slow in low visibility on the freeway, you are doing the correct thing. Driving too fast for the visibility is reckless and will get you a ticket for driving incorrectly for the conditions (assuming you don't get yourself or someone else killed).
When it's raining on the freeway, people in the faster lanes will slow down... to around 50-60 mph (instead of 70-80 mph). I don't think 50-60 mph is what Animats had in mind, though.
Op I believe means panic button full stop , pull over. As others have pointed out that's not possible in many places. Now those living the self driving car is easy dream will just say we have to make the roads safe for self driving cars, how many of you have lived through major highway construction in a city? And the political infighting and the eminent domain squabbles? I did in st. Louis, it took 5 years to agree on a half assed plan to add 1 lane each way to highway 40.
Or are you just gonna strap an extra lane onto the Brooklyn bridge cause self driving cars have to have it?
Also as the article pointed out human vision can see further than lidar esp in rain so what is a "comfortable speed for an entity with lidar vision is likely to get rear ended by an entity with human vision.
Heck if you go real slow on a highway and traffic isn't... you get rear ended at highway speed.