When my kids were 9 and 7 they were allowed to walk to friend's houses within a 4 block radius together, and the 9 year old was allowed to on their own. We knew where they were though, and it was only for arranged play dates. No walking the streets just because at that age, or going 15 minutes away. It's not something I was comfortable with for that age. But I happily let my kid stay home by themselves for an hour or so from about 9 years of age, as long as they were trustworthy. I have a friend who's 9 and 7 year old walk home from school and let themselves in to the house until their dad gets home about an hour later from work. She trusts her kids, and there has never been an issue.
Our school zoning area is not a big zone, and our kids are encouraged to walk to school, with or without a parent. In fact, by year 3 the school says that it's probably time for parent's to start letting their kids walk to school on their own, or with friends. My oldest was one of the minority when it came to walking to school with a parent at that age. Most got themselves to school. But, due to one road that we have to cross to get to school that repeatedly has cars running red lights I said no. The kids who walk to school without a parent don't cross that road. If we lived in a different street I would have been okay with it.
I also didn't just send my 9 and 7 year old without any type of preparation. We built up to it.
Oh, and we live in a city too. For whatever reason cities get a bad rep as being unsafe for kids.
Oh, and also....my mum was an overprotective parent so I didn't get sent out to go and play in the park for hours etc., but my DH's rule as a kid was 'be home for dinner.'