Whenever I'm walking, and I'm behind an old person who's moving really slowly, I'm always hesitant to pass them because I feel bad reminding them of their decrepitude. It's like, you know they don't want to be moving that slowly; they don't want it to take an hour to walk one block. So when you motor by them at three miles per hour, I feel that it's like saying, "See what you used to be able to do?!" and therefore reminding them of their impending death.
So I try to pass them at a relatively slow rate so they can feel like they haven't dropped off that much. So they can think, "Hey! That guy's half my age and he's barely passing me!", you know. Then, as soon as I turn the corner, and they can't see me anymore...it's off to the races!