Hazel McKay's poem paints a quick snapshot of a city at the end of the day. Cars "swarm" through the streets, all in a hurry to get home, while crowds of people fill the sidewalks. The word "swarm" is a good clue to the speaker's view: it makes the traffic sound like insects, busy and a little overwhelming rather than orderly.
The poem moves from movement to sound. By the fourth line, talking and honking blend into one wall of noise that "fill[s] the air." Then the last line steps back with a short, flat statement: "Tonight won't be quiet." There's no exclamation, so the speaker seems more tired or resigned than angry. They sound like someone who already knows the city won't offer any rest.
The mood is restless and crowded, with a hint of weariness. The short lines and plain wording feel like the rush itself: no pause, no calm, just the next sound arriving.