When I lived in San Francisco I took public transportation just about every day. Since moving to Portland I've used it much less. So I've made this transition back to car culture after a decade in bus culture, a transition that involves several types of mental accommodation. Being in public requires a complex set of strategies in order to get through it with your sanity intact.
When I was bus-savvy, I got acclimated to all the various casual violations of personal space that riding the bus demands. I got used to sitting directly beneath standing straphangers, for instance. I'm not comfortable being subjected to visual scrutiny. I'm not used to being looked at from above, or, perhaps worse, potentially being looked at from above. The social negotiation of riding the bus dictates that those who are forced to stand exact payment from those who are comfortably seated by being awarded the invitation to study the tops of their heads, look at the pages of the books they are reading, and obtain privileged views of female cleavage. When I'm sitting on the bus, I would very much like to know whether the person standing next to me is, in fact, scrutinizing me or any part of me or my belongings. I am prevented from gaining this knowledge, however, because another part of the bus social contract is that I am not allowed to glance up at the person standing next to me, certainly not to make eye contact. I am, however, allowed to gaze into the middle distance, at other passengers further up in the coach. The difference seems to be that one kind of looking involves a necessary radical head tilt, while the other involves only a slight movement of the head, or none at all. You can look as long as you don't give the impression of trying to see.
When seated, not only does decorum prevent me from rapidly looking up to see if the standing person is looking down at me, but etiquette also prevents me from so much as turning my head to glance at the standing person's crotch, which is, troublingly but tantalizingly, at perfect eye level. I have been on buses in San Francisco in which any number of potentially intriguing male crotches are positioned mere inches from my face, each asserting itself as a barely concealed carnal threat or opportunity, and not only was I unable to acknowledge the source of this carnal threat or opportunity, I was also unable to crane my neck to glance at the face of the crotch's owner.
This kind of situation sometimes made it extremely distracting for me to ride the buses in San Francisco. I'd disembark and feel as though I'd survived an exhausting psychosexual ordeal, some kind of primal drama taking place in an abstract space teeming with erotic signifiers.