Ofcourse tragedies like the murder of Sara Payne or Holly Wells or Jessica Jackson do happen. It is the fact that these terrible events are so rare that they make national news.
However I think many parents have the risk out of proportion. The chances of a child being murdered or sexually abused by someone they know is far higher than them being kidnapped, raped and murdered by a stranger.
Roads are far more dangerous, but if children aren't ever allowed to cross quiet roads on their own they will never learn.
I know someone who drives a secondary school aged child to school because she feels that eleven is too young to start being independent. Her eleven year old daughter is allowed less independence than my seven year old son. I would not allow my seven year old son to walk to school on his own, but has walked to the letter box on his own. (Ie. with me spying on him in secret)
I think she is emotionally stunting her daughter's growth.