Here is a 60 Minutes segment about driverless cars that I just finished watching about 3 minutes ago. Apparently they are also having problems with lots of things, like figuring out snow or rain, but Google wants to have those things figured out in 5 years. The Feds are worried about things like what happens if the human is napping or reading and does not respond in a timely manner to events.
The feds are dumb. Nobody with half a clue will license half-autonomous vehicles for exactly the reasons you list. What will get licensed will be fully autonomous vehicles with the liability wholly in the hands of the vehicle provider, and that vehicle provider will go "nope, we ain't driving in snow." People keep trying to make this issue a lot more complicated than it is and it just isn't.
Or they'll figure out how to make driving in the snow reliable enough to do it. Sticking RFID chips in those highway reflector things would go a long way, and after a few years of autonomous vehicles being widely deployed and not getting in accidents I'm willing to bet the DoT is agreeable.
Realistically I expect we'll see not a switch to autonomous cars but a continuoum of cars taking over the most difficult jobs as tech matures. ABS, cruise control, traction control, electronic stabilization, intelligent transmissions, brake assist, collision avoidance, collision breaking and countless other systems will ensure that by the time autonomous cars are ready to take over we will be driving cars with arcade controls preventing us from being stupid.
Yep. That Fed guy was thinking about today and not tomorrow. Notice how he was writing with a pen and paper. (I hate to admit it but I am pretty terrible at that nowadays.) That is why Google is currently road testing cars without steering wheels or peddles or drivers. If these cars are relying on humans not to mess up they will not be successful or approved. And when they do figure out ice, snow, rain, etc. imagine how much easier it will be for even a native let alone a tourist from Arizona to "drive" to Whistler. Ice and snow is a real bitch. Especially at night.