Even in highway driving you can lose traction in an instant if you hit a patch of black ice. Autonomous vehicles will need to be able to recover from a complete loss of traction safely. This isn't trivial - in fact it's probably the most complicated bit of driving I tend to do. Once you are sliding and your steering wheel becomes more of a suggestion than a command, the entire act of driving becomes a process of trying to coax the car off the road using a combination of steering, brakes, and even occasionally gas. I think it's possible for a computer to do this - but you can't avoid all slides just by driving slowly.

Then there's the plethora of other winter fun you run into with a vehicle: getting stuck (happens all the time on city streets) and all the techniques to get unstuck, going too slow and losing your momentum (and thus traction), having every indication you have traction and then discovering you actually don't (it's very easy to be driving at a "safe" speed and still slide through an intersection), white out conditions where you are guessing where the lane is... etc...

To be clear, I believe most of these conditions could eventually be handled by computers. I also believe a lot of people drive too fast in/on snow. However, winter driving is in no way simple. It's a problem domain unto itself, and one I've seen relatively little work being done on.