When should kids go on public transportation themselves?

These days, going on public transportation can be an unnerving experience for even some of us adults. There are a lot of characters that look a little suspect. Whether true or not, a lot of people are going stir some of our deepest, darkest fears of life. Still, I can only imagine what it would be like for small children to be on a public transportation system. I personally would not allow children to go on these public transportation systems themselves. It is just common sense that there are a lot of people out there that could be dangerous. These days, it is better to be safe than sorry. Eventually there will be a time where it is safe to go alone, but children I think should most certainly not go alone on a public transportation system. At least that is my two cents on the subject.