I have had two bad incidents, both involving cars. As soon as I could ride again I did. Accidents are most often freak occurences, sometimes there is something you could have avoided, but often it just happens. There is no reason to not ride as long as you can do it with out harming your healing body(Right now I am waiting for a badly sprained ankle, missed a step on a ladder) because I don't want to injure it further, already spent a week in a cast. But as soon as you feel OK and your doctor is cool with it you shoud start doing easy rides. It is the best thing for your body and mind. Your body to get it back into limber shape and your mind to get the confidence to go out and pedal again.