We raised our kids in a large city in America. Perhaps we just bypassed this trend but we totally let our kids play outside unsupervised. We set boundaries and taught them the necessary skills to do so safely but then we let the go and play. And we lived on a pretty busy street. There are age considerations of course and we had a large family for the time (5 kids) so there was a wide range of ages with the elder kids watching the younger ones.

Maye we are just outliers but I don't think the reason is necessarily cars.