All it takes is for one idiot, er I mean driver, on the highway to slow down briefly in order to cause a shockwave/ripple effect to propagate backwards and cause a momentary traffic jam. The Mathematical Society of Traffic Flow has reproduced the effect in controlled conditions, a circular 230 meter track with 22 vehicles at Nakanihon Automotive College in Sakahogi-cho, Japan.
Their study concludes by saying that in the absence of a bottleneck, “a jam is generated spontaneously only if the average vehicle density exceeds the critical value.” …which seems to be approx 25 vehicles/Km, at least on a freeway.
Traffic jams without bottlenecks—experimental evidence for the physical mechanism of the formation of a jam – New Journal of Physics, 4 March 2008
– via NewScientist
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