You also have to know them well enough to avoid the shady areas, not look muggable, not be female, not be small, not carry anything that looks remotely valuable

You also have to know them well enough to avoid the shady areas, not look muggable, not be female, not be small, not carry anything that looks remotely valuable, and only walk during daylight hours or else you're taking a significant risk (though all of that didn't save me from random verbal abuse when walking around the city or taking public transit nonetheless). And you have to be cool with places being totally filthy, and regularly cross 6-lane roads on a regular basis (yes, most of SF is like this). Plus you have to have a thick skin to see the abject horrors of homelessness and drug addiction everyday and convince yourself, "yup, this is normal and totally fine". Yeah, that was fine for me when I was a 20 year old, relatively large man who didn't know that the rest of the world has much more hospitable places for pedestrians and human beings generally, but no thanks today. Kinda depressing that at least from a walkability standpoint it's about as good as it gets for the USA.