I'm sure I read somewhere that the brain doesn't develop enough to accurately process distance information until after 8 years old which is why they say over 8 for walking to school and crossing roads unsupervised. Before that age they can't actually judge distance and speed well enough to determine if it is safe to cross a road with traffic.

I trust my 5yo to walk next to me in a carpark but I still make sure she holds my hand as I don't trust other drivers and I am also fully aware of how easily she can be distracted or lose concentration.