I live downtown. I work downtown. I expect that when I step out onto the sidewalk, there might be someone walking that same sidewalk in proximity, so I look out before inserting myself into the traffic.
People in cars or on bicycles do this all the time, although many of these might be faulted for only looking out for their own, to the detriment of the smaller traveller (sometimes the victim in this is a pedestrian). We don't tend to talk about pedestrian-pedestrian collisions quite so much, although I have dozens of near misses every month.
We all recognize the situation, but I'm going to illustrate with something that happened to me this past week. I was proceeding up the street after finishing work, on my way to do a little shopping just north of my office. A woman stepped out of a doorway and quickly assumed the speed of the pedestrian traffic and blended right in. The man who exited the same door after her, however, stopped dead in front of me to watch the woman walking away, never even glancing to determine if there was oncoming pedestrian traffic. I almost ran him over!
Luckily, I can put on a show almost as well as a professional soccer player (you know, the horrible falls suggesting that a terrible foul has befallen them), to I made a big braking and avoiding gesture that elicited a brief embarrassed apology from him. I suspect that it was as much guilt about being caught looking at the woman as it was about obstructing my path, but I'll take it.
Don't count on my continued agility, however. As I age, I am getting more likely to just trample you if you get in the way!