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First rule of ant traffic: no overtaking

Ever seen an ant traffic jam? Researchers studying ant traffic are beginning to understand why

In exploring their environment, ants create huge trail systems like motorway networks. Many researchers have remarked that we may have much to learn from the way ant traffic flows along these trails which seem to be free of the jams that plague our roads. After all, it’s quite possible that evolution has somehow optimised ant traffic.

But most studies so far have concentrated on two way traffic in which head on encounters between ants dominate the dynamics. We looked at one such study of bidirectional ant traffic not so long ago.

But what of one way traffic? Physicists and motorists alike have long puzzled over the tremendous complexity of behaviour that emerges in one way traffic flow. Who hasn’t been stuck in a jam with no apparent cause and that suddenly evaporates for no obvious reason?

Physicists have long known that these effects are closely linked to the density of traffic. Below some traffic density threshold, the flow is always smooth; but creep above this limit and all kinds of traffic chaos ensues.

Now Alexander John at the University of Cologne in Germany and few mates have studied the traffic flow along trails made by Leptogenys processionalis, a more or less average species of ant.

And what they found is quite extraordinary: the average speed of the ants remains constant, regardless of the density of the traffic. There is no transition to a nonlinear flow, at least not in the conditions that this group studied.

Let’s put that in perspective. Ant traffic flow is like rush hour traffic on the New Jersey Turnpike travelling bumper-to bumper at the 55 mph.

So what’s the secret? John and his mates aren’t entirely sure but they’ve found a pretty good clue: ants never overtake. Not ever. Instead they form into platoons in which all the ants move at the same speed. Increase the density of ant traffic and the platoons simply join together to form larger groups. This is how the velocity remains the same while the density increases. That makes ant traffic significantly different from other types of traffic in which congestion occurs, such as road traffic and internet packet traffic.

That’s neat but how have ants hit on this congestion-free solution? Almost certainly through the process of evolution which may well have selected for ants that use the most efficient transport systems.

There’s a lesson for traffic planners there, somewhere.

Ref: arxiv.org/abs/0903.2717: Traffic-like Collective Movement of Ants on Trails: Absence of Jammed Phase

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