With or without parent's supervision?

The post I was replying to specifically said he's scared that his kid could be hit by a car when going to school on his own

It's fairly obvious that accidents are the most common cause of death for kids My point is that parent supervision don't protect them from being hit or end up in a car crash or fall down from a bridge

p.s.: main cause doesn't mean it is common I believe it's not very common, it's catastrophic for their young body when it happens, but not common

Critical part is actually "going to school". Typical stats on accidents do not say where you were going, but at most where you actually were. My suspicion is that most vehicular accidents happen during play or free roaming, especially in an unsupervised kid group, not when actually going somewhere on their own.

CDC has data for the US only. The suspicion is that suburban development is the main cause, not walking to school or to school bus.

In actual cities the rates should be lower as roads are not mixed with recreational or pedestrian areas as much, at least in Europe. It is still the leading cause though. (WHO has good data.)

Quite a few accidents are also caused by biking...