In comparison, basically no self driving car could safely drive 100 ft during a snowstorm.
Getting self driving cars to be as good as humans during harsh weather (which is not that good I'll give you that) will take 10 years in my opinion (I say this as a grad student in mobile robotics).
Autonomous cars are already safer on ice. being able to individually apply abs to each wheel in micro second speeds of detection of change in terrain is far more accurate than any human.
As far as snowfall, for the vast majority of places, hi def maps and semantic knowledge of signage already exists, so as long as the car can localize, it can obey snow covered signs, and google and others are already working on filtering out weather "noise".
The problem isn't as intractable as it seems
Rough localization in a snowstorm (as in good enough to use prior knowledge of signs) or rain shouldn't be too hard. GPS still works in bad weather and lidar localization (scan-to-map matching) should be hampered but still usuable.
Seasoned New England drivers are idiots. Source: Am seasoned New England driver.
The idea that we can quantify all of the subtle little ways the human brain instinctively has gotten so good at driving cars is supreme arrogance.