The "cause of accident" discussion is basically irrelevant. By it's basic definition, an accident is not a controlled, planned occurance, so every precuation, safety measure, personal awareness, safe riding action doesn't even figure into the event. The most you can do is protect yourself, to some extent, from the outcomes of the event, and that protection is based on a sliding scale that each individual determines. I wear a helmet, but I don't wear knee and elbow pads when I mountain bike. I know I'll get skinned up, but my head has some protection.

The most convincing people for wearing helmets while biking are the nurses who work in the emergency rooms and trauma centers of your hospitals. Talk with them about what they see - it's pretty sobering.