# Sometimes a Slow Train Is a Good Thing

A traffic model incorporating subway and street traffic suggests faster trains aren't always good for congestion.

Unless you really like walking, chances are your morning commute is either by car or by public transportation, like the subway. Either way, city planners would like to reduce congestion and keep people moving toward their destinations, and you'd think the easiest way to do that is to just speed up the subways. But a new study suggests the opposite: In some cities, it might actually be best to slow trains down.

That result emerged when Emanuele StranoMarc Barthelemy, and colleagues looked at two cities, New York and London, and asked how streets and subway trains might work together to reduce—or create—congestion. Their approach is based on the mathematics of networks, a somewhat abstract field that's applied more and more to concrete problems like, say, designing streets and subway systems to move people around most efficiently.

But, Strano, Barthelemy, and their team argue, there's been a notable omission: Though there are many ways to get around a city, network scientists almost always examine just one at a time, likely at their disadvantage. Imagine New York at rush hour, where a commuter might take a mix of cabs, buses, and subway trains to get home—studying the street network or the subway network in isolation isn't going to cut it if the goal is to really understand how people get around.

### In some cities, it might actually be best to slow trains down.

To look at traffic and, in particular, congestion more holistically, the team first layered street maps over subway maps in New York City and London, incorporating street lengths, distances between subway stops, and the speed of subway trains relative to cars, in order to estimate travel times between any two points in a city. Assuming that most people would take the shortest path from point A to point B, and taking into account cars' and trains' average speeds, the team modeled where traffic would most likely pile up—that is, where travelers' paths were most likely to cross each other.

As the researchers increased the relative speed of subway trains in their model, they found that congested areas moved from the urban cores of New York and London to the outer edges of the cities, where subway lines ended. Presumably, the team writes, that's because those points are gateways between more car-friendly suburbs and an urban core best traversed by speedy subway trains.

In New York, the model suggests fast subways are a good thing, since the congestion in central New York City is so large that getting people in and out of those areas quickly is always better, even if it creates congestion in the suburbs. But the opposite was true in London, in part because of the Underground's more hub-and-spoke nature. If the goal is to even out congestion (as the researchers suggest it ought to be), it was best for trains to run between 25 and about 56 percent faster than the average speed of street traffic.

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