That is one of the best questions asked here in a long time
It really comes down to realistic risk assessment by the parent ( ie not giving into the media-fed fear), laid out borders to the child with the reward of those borders expanded if expectations of safety, trust, and common sense are met.
Here's a website I recommend:
http://freerangekids.wordpress.com/also an interesting article kind-of-related to your question
http://www.dailymail.co.uk/news/article-462091/How-children-lost-right-roam-generations.html
