London's Traffic Problems Won't Be Solved by a New Driving Tax

(Bloomberg Opinion) -- How do you get to work in the morning? If you live within one kilometer, or about half a mile, of a bus or train stop, there’s a good chance you take public transit. If you live further away, you’re much more likely to drive.

That fact matters a great deal to cities around the world facing looming problems of traffic congestion, from traffic jams to smog. The Covid-19 crisis offered a brief reprieve — congestion fell by 20% globally, according to data from the navigation firm TomTom — but traffic has since resumed the 2-3% annual growth of the pre-pandemic era. A few years ago, enthusiasts for ride hailing services like Lyft and Uber suggested they would solve the problem, but, instead, they seem to have made it slightly worse.

In London, Mayor Sadiq Khan recently proposed a solution that might occur to any professor of economics: to deter driving, make people pay more for it. The city pioneered that approach nearly 20 years ago by levying a congestion charge on central London. Khan has suggested replacing that successful scheme with another that would cover much more of the city. It could reduce pollution, encourage more people to use London’s underfunded public transport system, and help achieve the nearly 30% reduction in traffic needed if the city is to meet its goal of being carbon neutral by 2030.

But would it work? Unfortunately, that's not at all clear. The idea of steering human behavior through economic incentives rests on the assumption that a significant fraction of people really are making a conscious choice — in this case, to drive rather than taking public transport — in which case an extra charge might change their minds. The current evidence suggests that, in London at least, this crucial slice of the population may be quite small.

New sources of data can reopen old questions about traffic management. Even a decade ago, traffic policy was fashioned without a detailed understanding of the factors influencing peoples’ decisions to drive. This has changed dramatically. Researchers now have real-time driving data collected by navigation companies such as TomTom and Google Maps. That’s how we know that one kilometer can make all the difference.

Across 25 of the world's largest cities, a simpler empirical rule holds true: The smaller the number of people living within a kilometer of a bus or train stop, the more road traffic and congestion a city suffers. Because one kilometer takes about 10 minutes to walk, this reflects how many people in the city have public transport as a viable choice. Most people who live further away have few alternatives to driving. Small deviations from this rule appear in some cities with lots of bicycle use — in the Netherlands, for example — but other modes of travel play a minor role for most cities.

Why does this matter for London? Because, as physicist Marc Barthelemy of the Institute of Theoretical Physics in Saclay, France, pointed out to me by email, data for London suggests that very few people are currently deciding to drive out of what we might call “laziness.” According to Barthelemy, one of the authors of the “one kilometer” study, the fraction of commuters who drive in London is about 40%. That is roughly the same as the fraction of the population that lives further than one kilometer from public transit.

“For London, the share of car traffic is indeed governed by the density of public transport,” he says, “And it’s only a minority of people who prefer the car for reasons of comfort.”

If this is the case, then the impact of an extended charging scheme may well be marginal — on traffic, at least, if not on peoples’ personal finances. Even so, Barthelemy cautions that the fraction of people living close to public transport is only part of the story. What’s needed is far more detailed knowledge — through surveys and other means — of why drivers in London decide to take their cars. Some limited surveys do find more drivers among those living in outer London, or in areas with poor access to public transport. But survey responses notoriously depend on what questions people get asked.

Better knowledge of why people drive is the only way authorities can really tell whether the public transport infrastructure is the biggest problem, or if driving fees might actually make a difference.

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This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Mark Buchanan, a physicist and science writer, is the author of the book "Forecast: What Physics, Meteorology and the Natural Sciences Can Teach Us About Economics."

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