I didn't start using public transportation by myself until I was 12. It didn't kill or cripple me to wait a few years. Letting 9 year olds ride the subway alone is not the only way to give them responsibility or show that you trust them.

I appreciate that my mom was "overprotective" and didn't let me go places alone til I was older, even though I always complained, "but Mom, other kids are doing it!" And guess what? Other kids were getting abducted too, including a very smart/observant/mature schoolmate of mine. Fortunately, she managed to get away. So being "intelligent" doesn't make a kid safe from predators.

Let's also keep in mind that predators come in many shapes and sizes, it's not just the creepy guy who lives next door. They are men, women, young and old. In my town, there was a little old lady who killed over a dozen people and hid their bodies in her basement.