I used to let my DC and a friend (whose mother agreed and approved) go to the shop for sweets or a drink 'alone' aged 9. But I actually shadowed them. And there were no major roads to cross.
By age 10 my DC got the bus into a local town occasionally and went to a cereal cafe to play the vintage video games. they loved that little bit of independence and it helped them feel confident travelling into the city to go to secondary.
I agree OP that the parent should check first. But if there are no busy roads, and they stick together, and the adult watches them until out of sight and follows them if they're not back on time, then it's good for them.
One main reason (IMH and not scientifically tested O) that teens have such high rates of anxiety these days is that they are not exposed to fending for themselves at the age when the brain is ripe to develop these skills - between 7-12, playing outside, sorting out spats with mates, mending broken toys, bikes, making their own breakfast or lunch, taking short, safe excursions unattended by adults. These build self reliance, self confidence, resilience, problem solving. If we keep junior school children wrapped in cotton wool then toss them to the lions at secondary, it's no wonder they panic and feel overwhelmed and anxious and can't process the levels of stimulation and responsibility.