The analogy used to describe the nature of spiral arms as density waves compares them to what terrestrial phenomenon?

Answer

Traffic jams on a massive, rotating highway

The text likens the dynamic nature of spiral arms to traffic jams on a freeway, where vehicles temporarily slow down due to a bottleneck (the density wave) before speeding up again.

The analogy used to describe the nature of spiral arms as density waves compares them to what terrestrial phenomenon?
galaxyastronomystructurearmsMilky Way