ETA relevant links: https://youtube.com/@NotJustBikes https://letgrow.org/
ETA again: I glibly mentioned "being killed by a driver" but of course navigating the typical US built environment if you're under 16 or otherwise unable to drive is a miserable experience in a number of ways even if you survive it. Highways make pedestrian paths unnecessarily roundabout. Parking lots make everything further from everything else. Crossing major roads requires getting drivers to notice and stop for you (harder when you're short!), or waiting through interminable signal cycles, etc.
It may depends on where in the US you're talking about, but in my area none of this is actually true. Although lots of people believe it is.
I don't ever leave my kids (one that is under 6 and one that is well over 6, but not yet 13) alone in a car, since I don't want police or nosy passersby injecting themselves in a perfectly benign situation.