Question

Describe Density Waves in traffic

Imagine you are driving one such car. Describe what happens before you encounter the truck, as you wait behind it, and after you pass it. Address your speed, and the "density" of the traffic around you.

Answer 1

Density of Waves is one of the most successful theories which was proposed in order to explain the structure of spiral arms of Spiral galaxies. It's idea is that the denser parts of the spiral galaxies move slower than the individual stars and interstellar materials.
Similarly in our traffic jam parlance the cars normally fly down the freeway at high speed, but when they come upon the truck they must slow down to avoid collisions. A bird's eye view will tell us that the density of cars behind the truck is high compared to the rest of the freeway, and this region of high density will move with the truck for as long as it remains on the freeway. While the traffic jam behind the truck remains a feature that slowly moves forward, the individual cars involved in the jam are constantly changing. In other words the density wave behind the truck has a speed slower than the rest of the freeway. As one car manages to overtake the truck, another will come up from behind to take its place. The traffic jam remains, even though individual cars are only involved in it for a short period of time. The car that managed to overtake now faces a region of lesser density and thus it can move at faster speeds.