Have you ever reached the start of a traffic jam fearing the worst—road works, an accident, a fallen tree—to later discover no clear reason for the delay? Then you fell victim to one of the strangest traffic phenomena: the phantom traffic jam. This ghostly foe may seem supernatural, but can actually be predicted through the theory of shockwaves.
We can model traffic by treating it as a compressible fluid, the equations for which are above. In this case, x is the distance along a road (of infinite length), ρ is the density of cars at any given point and u(x)is the speed.
A car braking a little too hard somewhere will result in a small decrease in the steady state solution, where all cars are travelling at the same speed ū and the traffic density is constant. The car behind then has to brake slightly, as does the car behind that , and the one behind that, and so on, resulting in a soliton-like wave—aptly named a jamiton—travelling backwards along the road at a speed of around 12 mph (as calculated by traffic monitoring agencies).
More from Chalkdust
- Humbug sets the fourth puzzle. Can you solve it?
- Space-filling curves, cheating at cards and automated joke generation feature in our latest edition. Plus all your favourite puzzles & columns.
- Celebrate the launch of issue 11 by taking part in our puzzle hunt
- £100 of MathsGear goodies to be won if you can solve it
- Are you a torus? A cone? Ed Spheran? Find out by answering a maximum of five easy questions
- Grant applications, musical media and last-minute Olympics training are the topics readers have sent in to the professor's postbox this issue