Jam today

Mathematicians make headway in understanding traffic congestion.

Just as London drivers steeled themselves on September 6 for traffic chaos in the wake of a strike by workers on the Tube,as the citys underground railway is called,Britains pre-eminent scientific academy published a slew of timely papers. A special issue of the Philosophical Transactions of the Royal Society was devoted entirely to understanding and preventing road congestion.

These insights are welcome. At current rates,the number of cars and light trucks worldwide is set to double over the next 20 years,from todays estimated 900 million. Bashing out new cars is relatively easy; building new roads to accommodate them is anything but. Figuring out how to use the available road space more efficiently will thus be necessary to keep ever more cars from languishing in jams,and spewing out prodigious quantities of carbon dioxide as they do.

Scientists have been trying to bring order,or at least predictability,to motorway mêlées for decades. They assumed the familiar stop-and-go waves of congestion were due to the sheer volume of traffic. More recently,mathematical models have suggested they may actually be down to drivers behaviour. With cars moving fluidly in a tight pack,even a seemingly innocuous change of lanes may cause a tiny disruption which is propagated backwards for many miles.

Now,in one of the Royal Society papers,Jorge Laval,from the Georgia Institute of Technology in America,and Ludovic Leclercq,of Université de Lyon in France,finger timid and aggressive driver behaviour as the main culprit. To arrive at their conclusion,they looked at actual traffic on a 600-metre stretch of freeway lanes in Los Angeles,and another near San Francisco,and created a model to match the observed data. They found that vehicle speeds drop to zero if just a few drivers accept shorter distances between their car and the one in front,and a handful of others in the same lane prefer a greater gap,relative to the equilibrium spacing which in theory ensures a steady ride.

One way to maintain this ideal gap would be more widespread deployment of adaptive cruise control ACC,which enables partly automated driving. Some of these systems use the radar to keep a car at a set distance from the one in front. In another paper,Arne Kesting,of Technische Universität Dresden in Germany,and colleagues calculated that a 1 per cent increase in the number of ACC-using vehicles would free up 0.3 per cent of road capacity. Such systems have been around since the late 1990s but many motorists remain leery of relinquishing control to a computer. Until that changes they had better steel themselves for more jams.

The Economist Newspaper Limited 2010