A couple other things....
Walking to the end of your street with other kids and parents is not the same as crossing streets in an urban setting. If my 6 yr old had to cross a busy intersection to get to her bus stop she wouldn't be doing it without adults or a crossing guard.
A 6 year old needs more supervision than just a 10 yr old. I'm pretty sure I'm in the majority with this. 10 yr old is different. Heck I was out bike riding the town at 10.
Also....asbestos, lead paint and no seatbelts were once seen as safe in the 70s.
Also...laws are even more conservative. In most states you need adult supervision until 12-13.
False equivalency... Those things were scientifically found to be unsafe. The same is not true of unsupervised walking, which is probably a good deal safer than driving.