He knows what the bridge that he sits on is like and what it looks like from feeling it. He cannot see the people but he can hear them. He hears their footsteps but he cannot see their faces. We see what he sees, the sound of the bodies, the bridge but as people do not want to see him the view I show is my interpretation.
I have many images with people passing by blind people and not looking but for me this was stronger and required more thought.