To all those who feel that what yb613 said was offensive, or that girls hanging out on the streets is not a problem –
There is the concept of sensitivities. Yes, girls can say that there is no halachic problem with just hanging out outside, BUT that is not the right way. I am a girl, and I understand that if I want to hang out with my friends, I don’t do it outside. You can go to your friend’s house, or walk wherever you need to walk to, there is no problem whatsoever with walking to get to a destination, but the concept of just being outside and talking and laughing and being loud – that really isn’t so right. It definitely does fit into the category of tznius. On the other hand, if two or three girls who are not in a hyper mood want to go out for a nice relaxing Shabbos walk, and they can trust themselves not to get too loud or attention catching, I think that’s fine. Otherwise, it really is an issue. And it has nothing to do with boys seeing or not seeing, looking or not looking – it’s just not tznius, on the girls’ part.