The Anatomy of a Traffic Jam

For commuters and those in the delivery and courier industry, traffic jams are part and parcel of the daily grind. But what are the causes of jams? Now there is scientific research to add to the anecdotal evidence of everyday drivers everywhere.

It’s enough to make your heart sink. You’re on the motorway when the all-too-familiar cascade of brake lights comes towards you. Before long, you’re at a standstill. In this situation, you might feel for the trucker whose load is spoiling, or sympathise with the self-employed couriers whose jobs are nearing deadline; perhaps you’ll just feel the perennial frustration of the commuter. But you will probably also start to wonder about the cause of the delay: could it be road works, an accident, a police speed check?

Well, you won’t be surprised to hear that mathematicians have also asked these questions. There’s even a Mathematical Society of Traffic Flow. It is not just the frustration factor or the safety issue that make traffic jams a worthy research topic; increasing traffic flow is becoming an ecological and economic issue. When traffic is static, greenhouse gases are emitted needlessly; and for people who drive for a living, traffic jams can actually be costly. When vehicles are stopping-and-starting or sitting stationary with the engine running idle, they have reduced fuel efficiency. This can be costly for ‘the little guy’ who has to pay for his own fuel, like those driving self-employed courier jobs every day.

A Wave of Jam

Enter the Japanese scientists to the fray! In a study reported by New Scientist magazine, physicists have examined the anatomy of a traffic jam and been able to recreate the road conditions that can cause a seemingly spontaneous jam to appear. They have shown that even with cars moving at a relatively constant speed, a traffic jam can build from almost nothing.

The problem is that when the distance between cars varies just a fraction, the smallest responses by drivers create something of a domino reaction with tiny changes in speed having a cumulative effect. So when the long-distance trucker slows – on a hill perhaps – the self-employed courier behind him brakes a little, as does the coach driver behind him, and so on, until the stressed-out commuter five miles back finds himself at a standstill.

The scientists refer to the phenomenon as a shockwave. As soon as distance between vehicles begins to vary, each driver in turn must adapt his speed or distance from the other cars, and the necessary adjustment increases as it goes back from car to car. Remarkably, mathematicians have even been able to calculate that this shockwave travels backwards at around 20km/hr (about 12 miles per hour).

Small Change – Big Result

The painful truth is that these ‘shockwave’ jams are caused mostly by there being too many cars on the road. Furthermore, it means the smallest variation in traffic flow can snowball into a full-blown jam.

In February 2010, a small variation that had a big effect in the city of Brisbane was a family of ducks crossing the road during rush hour. The two adult ducks were hit first and their young scattered in panic. Many ducklings were killed as cars were unable to stop in time. As the motorists tried to negotiate around the birds running in all directions, the traffic quickly piled up and everyone in Brisbane was late getting to their jobs. A Courier-mail journalist was there to photograph the scene, and captured the surviving duckling. The whole incident was a tragic twist of the ‘sitting duck’ idiom and the ‘why did the chicken cross the road?’ joke. Answer: to screw up the traffic.

It seems traffic jams are a mathematical phenomenon, a logistical conundrum, and a modern headache that won’t soon disappear.

Source by Lyall Cresswell

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