It's good that you also talked about what happens 'after' the packet (not pocket) has left. It is really something the poem deal with, though? To be realistic, after a boat left has, do the cars or the drivers think about it?
You are right to say the speaker thinks about the smile, when the woman is not present. That's why the simile is used to describe not only the effects of the smile, but "the thought" of it. You can pay closer attention to the detailed description of the traffic. And the sound effects, too.