Last Christmas Eve in Topeka, Kansas, I patiently explained to my aunt that walking the sidewalks of New York City is just like driving the highways of Kansas. You are aware of the other people on the road only to the extent necessary to make sure they aren't about to wreck into you. The comparison is perhaps inadequate to describe how pointedly you must work to ignore the others on the sidewalks in the city as you walk them alone.
When walking the streets in a group, however, people gain certain advantages. Invisible bonds of gait, stride, and purpose join together two or more people walking as a group. The solo dancer on the pavement may choose to work with or go around the bonds between walkers, but only those who have created the ties may truly break them.
When I shared the metaphor with my aunt, she looked very sad. At the time I thought she really didn't understand what I was saying, that in fact it was as okay to ignore other walkers as it is to ignore other drivers, but now I'm not so sure. Walking alone lately, I have wondered whether I don't envy the Midwestern drivers their thick skins of metal and glass, which prevent the traveler from forming those idle, inconvenient bonds which hurt so much when they are inevitably broken. Maybe it is a little sad.