I know about termination with cause and without cause and at will employment but I find it hard to believe something like this would be legal. Even if the OP's boss believed she was endangering her life/safety by commuting by bike, I don't think it would stand. Otherwise, look how many people could be fired for driving to work in snowy or icy conditions. What the OP does on her own non-work time should be her business as long as it doesn't reflect badly on the company and I can't see how this would.