If you have the flu, what the HELL are you doing going to work? That's irresponsible. I have never had a boss that wouldn't chase my ass back out of the building if I came in.
As for light illnesses, if I'm well enough to be at work, I'll be better off if I ride. I'm much more alert and feel better after riding, even if I'm not 100% well, or haven't had enough sleep.
I don't really care about the weather. I will drive if it's truly miserable, which right now I define as more than about 3 inches of wet, slippery snow. Rain I don't care about. I wouldn't ride in heavy fog if all I had was run-of-the-mill taillights either, but with 3 Superflashes and a MagicShine tail plus reflective vest, I feel OK. I am about to order a 2nd MagicShine tail though.
I'll also drive if I need to go to the home improvement store and pick up a few hundred pounds of building supplies. It's silly not to since it's 11 miles to work and the store is 2 miles PAST work.
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Work: the 8 hours that separates bike rides.