Things They Should Study: gender ratios of different neighbourhoods

The other day I was sitting in my hairdresser's chair looking out the window, and I noticed that about 80% of passers-by were male. This morning, about 75% of the people in my section of the subway platform were female. I haven't made any other observations (I think this is going to lead me to walk around counting though), but it would be interesting to study whether there are patterns in different neighbourhoods or different places or different times of day.