Notice how the man just sits there, saying nothing. He notices every movement; each leg that brushes past his face out of every corner of his eyes. More likely than not, he's sitting there wondering where those people are going, and trying to figure out how his life got him stuck on that cold, empty sidewalk, surrounded by plenty of other human beings, but leaving him completely alone.
Just like a penny on the ground, the people unknowingly or uncaringly walk by him as if he isn't really there. He's sitting, low to the ground and wearing dull clothes, not saying a word to anyone. In a way, he is that penny. Some might stop, kneel down to say a few words to the man to pick up his feelings a bit, but most will make their ways to their board meetings, yoga appointments and their double mocha lattes without even thinking for a second about the person below them.
The man on the street is just an example of how people conduct themselves in public every day. It can be seen in the video above. Of course, those walking ignore the man sitting on the ground, but they also fail to take notice of each other. They seem like mindless zombies, only focused on where they are going and not caring about the others in existence around them. We often pass a number of fellow travelers everyday of our lives, but never acknowledge them.
In the days of top hats and poofy, uncomfortable dresses, it was common courtesy to say "Good day" to others in passing. Unfortunately at some point in history, we lost that sense of kindness and respect. It has become habit to strut around as if nothing else in the world matters but our own concerns. Without human interaction, there isn't much point to life. Without it, we all may as well be pennies on the sidewalk. We would notice each other just as much, but our existence would be worth more. Try saying hi to a stranger today. It's not much, but it's a good start.