It was pretty warm this morning, at 37 F, as I rode to work. But, the visibility was somewhere under 1/4 mile due to the fog.
Add to that the condensation on my glasses (or on the windshield of a car) and visibility was even more limited. I ran both of my 9-LED flashing tail lights, along with both the flashing headlight and steady headlight, and still felt invisible.
This fog would probably seem minor to Meli and her friends out in San Francisco, but we don't have a lot of fog here in Denver, as a general thing. So, people have a hard time adapting their driving to the conditions.
Kinda makes one nervous, on a bike.