Tuesday, April 01, 2008

Shockwave traffic jam recreated for first time

Often when I talk about agents and Agent-based models I give the example of cars on a simple road, with each car being an agent. Each car follows a simple set of rules: it slows down (decelerates) if it sees a car close ahead, and speeds up (accelerates) if it doesn't see a car ahead.

The model is simple but demonstrates how traffic jams can form (emerge) without any accidents, broken bridges, or overturned trucks. Furthermore it show how no "centralized cause" is needed for a traffic jam to form.

The mathematical theory behind these so-called "shockwave" jams was developed more than 15 years ago using models that show jams appear from nowhere on roads carrying their maximum capacity of free-flowing traffic – typically triggered by a single driver slowing down.

Below is a YouTube movie of the NetLogo Traffic Basic Model.

The theory has frequently been modelled in computer simulations, and seems to fit with observations of real traffic, but has never been recreated experimentally until now. Now a team of Japanese researchers has recreated the phenomenon on a test-track for the first time by putting 22 vehicles on a 230-metre single-lane circuit (see the movie below).

Drivers were asked to cruise steadily at 30 kilometres per hour, and at first the traffic moved freely. But small fluctuations soon appeared in distances between cars, breaking down the free flow, until finally a cluster of several vehicles was forced to stop completely for a moment. That cluster spread backwards through the traffic like a shockwave. Every time a vehicle at the front of the cluster was able to escape at up to 40 km/h, another vehicle joined the back of the jam.

The full article can be read in New Scientist (click here).

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