We live on a very busy street (couple thousand cars a day going 30-50mph), with a lot of foot traffic (couple hundred people walking back and forth to the university, shopping, homeless, etc.), 12 level 3 (high risk) registered sex offenders with 1/2 mile. (We actually live in a very safe neighborhood as far as the city is concerned, but the city doesn't have actual 'neighborhoods' ALL of the residential areas have 30mph speed limits and no sidewalks much less cul de sacs). So I don't let my 8yo play in the neighborhood. He can play out back whenever he wants, but out front one of us needs to be with him.
But 3 years ago when we lived an hour north in a family friendly area I would let him play, and when we're in other areas (the mountians, visiting family in the midwest, in small towns all over the country, or in countries that are family friendly -like Italy-) I'll let him play with other kids quasi or completely unsupervised. It REALLY depends on the area. I'm all for free range kids... but also for not being stupid.