When I am walking by myself in big cities that may be a little sketchy (any Central American capital), I always make sure to stay in populated areas. If I find myself walking block after block and seeing no one I get nervous, "Is this a bad neighborhood? Should I not be here?" and then I turn around.
I feel much more comfortable on a crowded city street, filled with hustlers, pickpockets, and hookers, than vacant ones.
The entire time I was in Milwaukee with my father (see below post) we saw about four or five people on the street. This made me uneasy. I had never heard of Milwaukee being a dangerous place, but where was everyone? It kind of felt like an Armageddon, End of days, post-apocalyptic kind of thing. There were plenty of big buildings, Dunk 'n Donuts, offices, museums, etc., but no people to fill them.
Someone call Homeland Security, the people of Milwaukee have all been abducted!