The US is about thirty times larger than Poland and much of it was developed after the wide adoption of automobiles. Few children live within a mile of their school here, and of the ones that do, there are often not walkable routes to get there. Many roads in the US have no sidewalk and are intended only for vehicle traffic.
Is that really true? 80% of Americans live within large metro areas rather than rural areas.
Look for the percentage who actually live in the cities, vs. the metro areas.
And when I lived in the Denver suburbs, my brother attended a couple of county schools. The junior high was at least a couple of miles away, the high school more like five. Do not underestimate American sprawl.
I am one of the few parents that trusts their own kids to walk a short distance to school by themselves. I have received a few comments from other parents implying negligence for trusting my kids.
I have doubts that it's busy roads that changed this behaviour.
Part of it is the spread-out car culture of the US, where virtually the entire country is so car-centric that walking anywhere is prohibitively difficult and/or risky. It's kinda self-reinforcing - walking is awkward and discouraged, so almost nobody does it, so there's no incentive to fix any of the walkability problems, like missing or neglected sidewalks, bad traffic light behavior, huge parking lots everywhere to cross, etc.
But the bigger part IMHO is the stranger-danger paranoia. Google the free-range parenting movement here for a few disturbing stories. The media hypes up stories about rare child molesters who kidnap children off the streets for better viewership numbers, then people start to think that kidnappers are everywhere and you don't dare take your eyes off a kid for a minute. This makes its way into policy and expectations. Add that into the common viewpoint here that if you have a child, they must be the most important thing in your life and your entire life must be centered around improving the kid's life. You get a society where letting a child, or even teenager, walk to school seems so outrageously dangerous and irresponsible that kids may sometimes be picked up by the police and returned to their parents even though nothing is wrong, and the parents may sometimes even be investigated themselves for their supposedly wildly irresponsible behavior.
When I first went to school, my mother told me not to accept lollies, gifts or rides from strangers. The same fears existed, but people tempered those fears and kept paranoia in check.
What has most probably changed is the social attitude which has become far more judgemental and much more intolerant. I've given a lot of thought to these social attitudes and I see today's society as closer to the Torch and Pitchfork scenes in Beauty and the Beast (think of the feminist attack on Matt Taylor by The Verge as one of many examples of extreme verbal aggression by the masses). This mob intolerance has become popular and frequent in the last 10 or so years.
The media and political classes just creates an environment where it seems more dangerous for their own benefit. People who are constantly afraid buy more newspapers/click more news links and will vote to reduce their own rights. All the better if the dangers are essentially imaginary - the gravy train can keep running without worrying about figuring out something to do that actually solves the problem, or the problem going away once it's solved.
Most german friends I know of started earlier than 8:00 too (most started at 7:30). And yes I'm talking about actual lessons starting at that time - not "day care". Even though from our point of view there seemingly wasn't really a difference between those anyways...