In cities, I'm sure cycling is better than transit or crowded sidewalks. But if you compare it with a private auto having one occupant, it's probably no better.
Actually a car would be the safest place to go besides your home. The car windows are closed, you have the ventilation system on which the incoming air goes through the cabin filter first; besides all of that the virus isn't going to travel that far anyway, and it's the form of tiny water particles, what is known as an aerosol, it's not a gas. If you're out riding the bike you have no enclosure and no filtration system, the only thing protecting you is distance and maybe a face mask.
Speaking of face mask, the only really effective filter against a virus is a HEPA filter, even surgical face mask are not HEPA filters. What this means is that unless the face mask is a true HEPA filter that 5% of the virus will pass through those other masks, if you have a HEPA filter only 0.03% will make it past the filter. What's really bad are those homemade jobs people are making and some are selling, these will allow about 25% of the virus to past through. Even the websites that tell you how to make a N95 mask will NOT keep you safe, there is a 5% pass through rate with those.
So your best defense is distance, or in a closed up car, or in a closed up house.
