If you ride conservatively (use lights at night, use good judgement at intersections) and stay away from buses and trucks, the exercise vastly outweighs any risk.
This is very dependent on the environment. I would not be so confident saying this about the USA in particular.
If you can stick to at least half-decent dedicated cycling infrastructure separated from cars, sure. If riding alongside traffic, I wouldn't be surprised that, depending on the exact route, the gains wouldn't outweigh the risks.
Generally speaking, operating a bike safely is considerably more difficult than a car and the margins of error are tiny.