I have witnessed a completely mentally capable (not drunk) human driver slowly (walking speed) drive directly into a ditch, missing the intersection by 10 meters, simply because of a smooth and even layer of pure, fresh white snow everywhere and overcast but still pretty bright (thin layer of clouds) day giving extremely even lighting. Human brain's vision system simply could not do the required 3D mapping from the stereo images captured by eyes because everything is evenly colored and the shape of the ditch blurred. At the time I was developing "solid state lidar" (or more accurately, 3D time-of-flight) stuff and it occurred me how funny it is that the 10€ sensor we were using would trivially see the ditch. (At the same time, Tesla released "stunning" demos of their camera system producing point clouds, and I immediately spotted how the road surface itself was completely missing from the point clouds, which is understandable since it's a smooth gray surface. But I'm sure they are doing better already.)
It is also surprising how many human drivers have serious troubles staying on their own lanes on narrow roads. I mean, of course, with no upcoming traffic of course it's normal to slightly deviate from the lane if the road is narrow enough. But what surprises me time after time are those who keep driving in the middle of the road even with traffic to opposite direction. When two with the same sort of brain damage cross each other, an accident is very close.
Also very common pattern seen here is completely ignoring pedestrians and cyclists. Rules about this are very clear, they are road users just like cars so you blink and overtake them as you would overtake another car, giving plenty of physical separation (something around 2 meters, exact number here is irrelevant). If you can't due to other traffic, you slow down and wait for a safe occasion. Instead, many people just drive some tens of centimeters from pedestrians and cyclists at 100km/h simply because they are not interested about following the rules.
So while narrow roads and difficult weather are very difficult to self-driving cars, they surely are difficult to human drivers too.
Snow specifically makes it difficult to spot where the road is; human only has non-calibrated pair of eyes not "designed" for photogrammetry applications, with a pretty powerful neural network system which works surprisingly well for the crap input it gets, but sometimes fails miserably.
I would definitely add multi-source sensor fusion to that mix, to have better chances of knowing where the road is (or even better, where my lane is) in presence of a lot of snow. As a human I still have a pretty powerful brain, but I can't connect to satellites for positioning, and I cannot shoot lasers and measure distances with my eyes, and I also cannot send and receive radio waves. Not even ultrasonic like bats.