There’s probably very little weather that is safe for cars but unsafe for bikes. Uncomfortable, yes, possibly extremely so. But you can bike in a downpour so severe that it’s unsafe to drive specifically because you’re not in a 2 ton deaths machine.

Maybe a severe enough snow storm? Even then we’re in Goldilocks territory for the storm to be unsafe for bikes but safe(ish) for cars.

The biggest factor is that people simply will not get on their bikes in severe enough weather. At least not in most places. Maybe in the Netherlands they’ll bike in a blizzard.

Safe for cars/bikes, or the passengers vs the bicyclist?

Hail comes to mind. Lightning possibly (I believe cars are much better insulated against lighting strikes). High winds could easily push bikes around / knock them over where cars just keep going.

We drove our van through a forest fire (Cedar Creek Fire - a BIG one) and got a bit of smoke, but otherwise, just fine. No way would I have attempted that on a bike - the increased aerobic activity alone (to say nothing of embers / ashes / etc) would have probably caused crazy amounts of smoke inhalation / death.

And there is a reason drivers hate SOME bikers - here in CA, many simply refuse to follow the rules of the road. My light turns green, and 5 seconds later, some biker comes rolling along in the perpendicular direction - I almost hit him. This kind of stuff happens over and over. I am very fond of bikers when they follow the rules - I bike sometimes too.

Hell, we organize championships: https://www.youtube.com/watch?v=VMinwf-kRlA