Just as the novel coronavirus has spread from person to person across the world, so too does traffic propagate through highways and city centers like a contagious disease. From a single crash, congestion ripples through a city, and now scientists have the models to prove it. Researchers in Australia, Iran, and the US have modified a common model for mapping the spread of disease to show that it also works for describing the spread of traffic jams—only in this case it’s cars infecting each other with congestion instead of people infecting each other with a virus.
Weirdly, they found that in six distinct cities—Chicago, London, Melbourne, Montreal, Paris, and Sydney—traffic spreads quite similarly. “We can calculate how fast congestion spreads in a network, and that is actually independent of the geography and topography of the city,” says University of New South Wales engineer Meead Saberi, lead author on a new paper in Nature Communications describing the work. “It could be anywhere in the world, and the dynamics of the spread is very similar.”
More later on how that could possibly be—but first, let’s talk about those models. One way to characterize the spread of a disease like Covid-19 is known as a susceptible-infected-recovered model. Susceptible means the group of people who haven’t gotten the disease before and can now get sick; infected means those who are sick now; and recovered means those who’ve beaten the illness. Because the recovered are now immune, a pandemic tends to wane over time, as the virus has fewer and fewer potential hosts available to infect.
Adapting this model to characterizing traffic, the researchers looked at “links” instead of people, which means the physical roads between any two intersections. (A four-way stop is technically two roads coming together, but each direction counts as one link.) And instead of studying biological symptoms like coughing or fever, they studied traffic congestion, aka the jams in which cars slow and back up as a congested mass. “We have three different kinds of links in the network,” says Saberi. “Links that are susceptible to become congested, links that are congested, and links that have been congested and now they’re recovered.” So it’s the same analogy, he says, but “from a traffic perspective.”
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