we now live in a culdesac, and I am firm believer in encouraging independence, playing outdoors, go and knock for your friend yourself and all that stuff. (other parents think I am too lax)
oldest was allowed to play outside without me at 5 (different house, no cars, I checked regularly, had to curb his wings at next house)
second was allowed to once we moved here aged 6 - (previous house had busy road, I would cross her over and then she was allowed to go to corner shop, I watched out for her and crossed her back)
youngest is nearly 5 and not yet allowed to. I also think she is less reliable than dd1.
School lets them walk too and from on their own in year 4 - aged 8. dd1 has just started year 3 aged 7 and I would be happy for her to (not every day)
very hard to comment as depends on your road, your neighbours, your child and so on.
We have school friends who live about 300 yards away, across a dead end road and a grass patch. I suggested to mum that my dd and her dd could knock for each other (and we could text if she liked to know where they were). dds are 7. She was horrified, not even her 9 year old is allowed to do that, as she can't cross the (dead end) street safely. hmmm, maybe you should teach her then??