[to3m]

The comparison doesn't strike me as a very good one. There are no good, quick, easy ways of measuring how far from a cyclist you will be when you pass them, or your blood alcohol content (which is anyway only a crude proxy for level of intoxication, and besides is commonly measured by alcohol in one's breath, which isn't quite the same thing). That's why people are advised to err on the side of caution.

When it comes to speed, on the other hand, every car comes with a speedometer. Erring on the side of caution would not be a crime (the speed limit is a limit, after all, not a goal...), but it's far less necessary.