IMHO, if the conditions are bad enough that I can't trust OTHER cars to not hit me, then I shouldn't be out in them in a car either. Here are the conditions in order from good to bad, IMO:
- roads are good, ride the bike
- roads are worse, ride the bike, look out for cars
- roads are bad enough that cars can't be kept in control - stay home.
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Work: the 8 hours that separates bike rides.