Roads where bicycles are allowed but dangerous to use

Currently (sept 2011), there is no consensus about tagging dangerous roads for bikers. First, it is some personnal judgment - some would say "subjective stuff". What "Person A" biking with children considers as "dangerous" could be considered as "safe" for Person B driving alone with an mtb, etc.

I know two approaches about this topic : create a scale with numbers or values like "cycle:hazardous=1..5" or "hazard:bicycle=dangerous" (see the taginfo tool for some statistics). But this solution remains subjective even if you provide a detailed description for each value.
Or just describe physical attributes precisely with mapxspeed, width, amount of lanes, tunnel, etc then let software applications or personal judgment take their own decision about road dangerousness.

You can also search on the mailing lists archives (searching keywords like "bicycle=no", bike+hazard, etc) and check the different ideas/proposals around this question (for instance, this one). If you are convinced by one of the proposed solution, just use it. Even better, you can create a proposal on the wiki and call for a review on the main mailing lists (talk@ or tagging@openstreetmap.org)

Don't use "bicycle=no" which is only for legal restriction.