When I lived in Warren, Michigan in the 1970s I would rotate commuting by automobile, public bus or bicycle. My safety rule for snow was: No bicycling if more than one inch of fresh snow. During the 1990s I visited Boston and was amazed to see bicyclists older than me commuting in any amount of snow.
But I've learned, more than once, that we have a winter slide here in San José, California too. I've had to, since I no longer own an automobile, and public bus rarely goes where and when I want to. The worst months for it are: November and December. It's fallen leaves mixed with rain. Being aged now, I'm more careful – during our first winter storm(*) a few weeks ago, I was actually too careful and skidded down while stopping for a motorist coming out of a driveway who had probably already seen me and was waiting. Thanks to Easy Racer's recumbent bicycle technology, I only got a skinned elbow that was very sensitive for a few weeks, instead of another broken collar bone. No incidents occurred during my commute to or from the Haines Swim Center in Santa Clara this morning.
By the way, wet fallen leaves is another reason why seasoned bicyclists don't get too excited about bicycle lanes. Unless you have your own portable bike lane.
(*) Instead of storm, readers east of the Rocky Mountains should read rainy day. The word storm means something completely different to you.