We let our very-capable son and his less-capable-but-bright and more-experienced older sister freely wander the neighborhood on bicycles when they were 5.5 and 7, respectively. Worked out fine so far.
Varies by neighborhood, though. Our current one's busy-body and kids-only-play-with-parental-escort enough that we had a couple neighbors stop by to warn us that they'd seen our kids several streets over, thinking they'd gotten away from us. Not quite busy-body enough that anyone called the cops (I suspect we were right on the edge of that happening, and maybe just got lucky). Our last neighborhood had wonderful mixed-age "gangs" of kids wandering around playing all the time, and it would have been entirely safe there. That was a much younger neighborhood (in terms of both the ages of the houses and the average age of residents) than this one (not sure whether that's related), and, I suspect, there were some class issues at play (the other had a very high-prole character to it, in Fussellian terms, while this one's 100%, gratingly, middle-class as hell)
As for chances of assault, your main worry by a country mile should be cars, not predators. All forms of attacks on kids by strangers are incredibly rare. Leaving your kid in the company of a specific adult or set of adults is far riskier than letting them walk to the park (yet people do that all the time). Shit, statistically siblings or cousins are far "scarier" and worthy of concern, in that regard, than the risk of regular walks to a park 2 blocks away.