My best advice would be to never bike alone if you can avoid it. Too many "unforseen" events can happen and if you are alone - you may be put in a dangerous or life threatening position. If you have a buddy or friend with you - they can get help, call for help or assist you if you are hurt or unconscious. When you are alone - that is it - You are alone! If you do bike alone please tell someone where you are going and when you expect to be back(and stick to your route).
*also carry a cell phone if possible.