Accordion phenomenon on the highways

Published: 2018-02-26
Words: 432

The formation of accordions on highways is characterized by what is called, in fluid mechanics, a change of state of the flow of vehicles on the road: it is the transition from a "laminar" flow, to a "turbulent" flow.


Its main consequences are: 1 / the increase of the "resistance" of the road to the passage of the vehicles, which causes the decrease of the flow of the road (number of vehicles per unit of time), as well as 2 / by the presence of stationary vehicles on the road significantly increases the risk of collisions.

In the "laminar" flow, what characterizes mainly the interactions between the vehicles, is their small number. In other words, a vehicle in such a situation moves in a way and at a speed that does not seem to be influenced by the presence or the number of vehicles around it. The flow of the road is then maximum.

In the "turbulent" flow, on the other hand, it is the interactions that will mainly determine the behavior and speed of the vehicles. The situation is reversed, and these interactions induce a propagation of the accordion. We indeed note that a "wave" of slowdown seems to go back the road in the opposite direction of the movement of the vehicles.


This phenomenon has been studied extensively for decades, but without being fully understood, nor provided with an adequate solution to eradicate it.

So, but what causes them, and how to get rid of them?

The answer is simple: the accordions are created and then feed themselves only on the difference of flow of vehicles at both ends of the accordion.

That is to say that an accordion is created faster upstream, it is unlocked downstream.

This is to say that drivers re-accelerate more slowly when they leave the accordion, than they do brake when they enter it.


The total and definitive suppression of accordions requires the training of drivers to: 1/ slow down from far away, for a long time and gently, when they enter an accordion; then 2/ re-accelerate faster than the way they braked, when they come out. That's all.

For a concrete and interactive visualization, see the accordion simulator by the German researcher Martin Treiber.

Notice that no accordion is formed nor can survive if: "Max Accel a" ≥ "Comf Decel b"

Article written in response to:

Another solution to this problem is provided by: CGP Gray - The Simple Solution to Traffic (5:13)

And a real-life visualization of the phenomenon: New Scientist - Shockwave traffic jams recreated for first time (0:39)