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Reading this while stuck in traffic? Fast trains may be to blame. Slowing the London Underground could cut overall congestion in the area.

We’ve used mathematical tools from network theory to study transportation networks for a long time, but mainly looking at one mode of transport in isolation. That’s not enough, says Marc Barthelemy of the French Atomic Energy and Alternative Energies Commission in Gif-sur-Yvette. “Changing one network can have an effect on the congestion of the others.”


Barthelemy and colleagues studied these effects by mapping out the road and subway networks in London and New York City and simulating travel at different speeds. They found that the ratio between the average road and rail speeds can have a critical impact on congestion.

If the subway speeds up while road speed stays fixed, for example, more people on the outskirts will use trains. But these commuters often have to travel by road to get to the nearest station. The result is more congestion overall, as increased traffic at the outskirts outweighs smoother flow in the centre.

“If you want to give more access to more people, then increasing the speed of the subway is not the best solution,” says Barthelemy – increasing the number of train stations is often better. New York City is an exception, however: congestion here is so bad that speeding up trains does help.

The new model lacks some important details, says Michael Batty at University College London, such as the capacity of network links rather than just their speed.

But he thinks Transport for London, which oversees the city’s transport networks, should take note. In particular, the results suggest that Crossrail, a new high-speed line being built east-west across the whole of London, could increase congestion at the outskirts. “Their analysis could potentially say something about that,” he says. “It seems very useful for future network design.”

Reference: arxiv.org/abs/1508.07265