If you're using laser range finders to localize yourself (like Google, and all the other efforts I'm aware of; veen may know a counterexample) and don't have to worry about tracking other traffic you can just slow down to compensate for the unusually noisy measurements you're getting. Vehicle-to-vehicle communication will make the tracking problem much easier. That said, I'm betting everyone punts and says "drive yourself in snow or heavy rain" until most cars on the road are self-driving.
So there's three main issues that snow causes for autonomous vehicles: - It (partially) covers lane boundaries - Phantom objects - Black snow The easiest way for autonomous cars to find their place on the road is to find the lane boundaries and use them to center the car. Snow-covered roads make that impossible. Even cleared roads often still have snowbanks on the sides, which produces noisier data. As a result, the car sometimes detects objects that aren't there, called phantom objects: Lastly, there's black snow which is really hard to detect because it's so transparent. Solutions might be V2V communications (platooning, one car leads the way and the rest follow) but it needs to be very reliable. Slowing down won't help on snow-covered roads, but it might help on (partially) cleared roads. You're right though, it's a problem that will most likely be avoided as long as financially makes sense.
Figure 12. Example of pitching combined with small pose estimation errors: a The reading of the center beam of one of the lasers, integrated over time some of the terrain is scanned twice.; b shows 3D point cloud; c resulting map without probabilistic analysis, and d map with probabilistic analysis. The map shown in c possesses a phantom obstacle, large enough to force the vehicle off the road.
But human drivers don't know where the lane markers are if they're obscured either, so staying on the road, in roughly the right lane, isn't worse than a human driver is going to do. Alternately, if they become widespread enough, sticking rfid tags or something in the lane markers to help robots out isn't unreasonable. Working without infrastructural support is cool and probably necessary to get the technology out the door, but if we're all riding robot cars we can insist that the infrastructure adapt.