As I've posted elsewhere, my son has played outside, without my constant supervision, from an early age. And we live in London, though on the outskirts.
He has strict ground rules and has, IMO, really gained from having this independence. He has made lots of local friends, he has a great connection with the area, he gets lots of fresh air and exercise and his fun is not (always) dependent on expensive toys and technology.
When we moved house, we deliberately chose a quiet area where it was commonplace to see children play outside. A few years on, we are now at the stage where my son wants a bigger area to roam around in. He's nearly 8. Scary for us to give him more freedom, but letting him go is a progressive thing. He's adhered to our rules, pretty much, so now it's`time to give him more slack. Dh and I have been racking our brains about this recently. We fear acccident and abduction, as any parent does. But ,IMO, it's foolish to do all the letting go when your child reaches the age of 14, 15, 16.
I really agree with janh's post.
As for the fishing and dual carriageway reports, yes, on the surface the risks seem great, but, as Zoya says, who knows the real story? Looking at the children who play out in our area, hundreds and hundreds of them every day, reports of fatal accidents or abductions are, thank goodness, virtually zero.
IME, early involvment in petty crime and mixing with the wrong teenage company is a far bigger, though less life-threatening risk.