I live in Brooklyn and there's kids roaming around after school all over. Unless they're toddlers they know how to not get hit by a car.
I wouldn't be surprised if our parents' generation were also considered over-protective by previous standards. This just feels like a "when I was your age" type of thing.
And, of course, there are many degrees of protectiveness. It is a spectrum. I see some parents where their protectiveness is much greater than mine and it seems obvious to me. At the same time, I'm sure other parents look at me and say the same thing. It's not a one size fits all situation. Parenting, as a whole is, of course a matter of personal values.
But if you widen roads and make other improvements to accommodate traffic then the graph would be more linear.