The simplest explanation would lie in the juxtaposition of the two "couples" - each defined in contrast to the other. The "grungy" workers symbolize a Marxist proletariat, while the beautiful young people riding in the open Mercedes clearly represent the bourgeoisie.
An interesting "flip" on the theme comes when Ferlinghetti compares the older of the two scavengers with "grey iron hair" to the blondness of the younger one as if the latter were not of the working class long enough to have been "hunched" by his constant working on behalf of the Mercedes people.
Later, when the scavengers look across the gulf while killing time at the red light, it's as if the hope to which Ferlinghetti refers is somehow greater for the younger garbage man as he appears more like his counterparts in the roadster than the older one does.
In the end, however, the hope for either scavenger is an illusion. Equality in a capitalist democracy is non-existent. Just as on the farm, some pigs are more equal than others.