I don't think real danger completely explains it. I live in a completely tame neighborhood. Kids definitely walk and bike to school. Yet, there is also a much higher prevalence of kids being driven to school than when I was a kid.

And I grew up in true car culture: The coolest kid had a Cobra jacket, which meant his dad worked for Ford. That kid walked to school.

One difference is that there's a lot more car traffic in neighborhoods today.