You would rate the stopping distance of the bike at, say, 15 mph, on a dry surface. Have the standard set quite loose, and give fines from it to bike co-ops or something to support bicycle maintenance for low-income people.
And, the law would be written to be so weakly enforceable that it could only really be used if a bike didn't have an applicable brake at all, or if the cyclist was involved in an accident due to failure to stop.