First it should be noted that this senryu is a gentle type ~ not a harsh joke or statement which is the other more common type. It has wabi-sabi, when seen externally as an image of an elderly blind man seated solitary amongst a busy and indifferent city crowd. But it also hints using the key verb "observes" the blind man's auditory perception, which brings in an element of mystery and wonder. One can hear the buses groaning past and wonders at the vast, mysterious way in which the blind man "sees" them. The last line makes this inner seeing more acute "pass on, one by one ~". In this aspect, there is no sadness but only wonder.
This is a tremendous senryu, Narayanan!It carries great power in the choice of expressions. I fully endorse Shyam's analysis of it. It is quite thought-provoking...there is something metaphysical about it, too!