SO HERE I am, sitting in stationary traffic, doing my best to blend in and act normally; hoping my cover’s not blown. Slyly I look around. The Volvo on my left contains a man picking his nose; in an Astra on my right, another man is eating it.
After what seems like hours, we start moving, but before we hit normal speed, a man cuts in front of a caravan, which slams on the brakes. The driver behind follows suit, and now it’s my turn. I honk (I’m ‘in character’), and am summarily flipped the bird.
Glancing behind me, I see a veritable stream of traffic has ground to a halt, and, while the cars in front of me are moving again, as each person takes a few milliseconds to speed up in response to the acceleration of the driver in front, the cars a few miles behind won’t start moving properly for a good half-hour.
Back safely at home, I look up traffic jams, and discover what I observed was a ‘ripple effect’; where small disturbances in speed lead to strings of cars having to slow down and speed up, which causes delays because neither cars nor people respond instantly to inputs; reaction times delay the waves of acceleration and deceleration which characterise traffic jams.
Scientists have actually recreated this by getting 22 cars to drive around a large circle at 30 miles per hour. At first they flowed smoothly, but because drivers did not keep a uniform speed, ripples of slowing and speeding up eventually led to the kind of congestion which would usually be seen on a busy motorway. A video of the experiment can be found at newscientist.com/article/dn13402
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