Why Building Roads Creates Traffic

June 9, 2011 at 2:38 pm 3 comments

Posted on Monday June 6th by Eric Jaffe for The Infrastructurist – Ameica Under Construction

In the transportation world our intuition can lead us astray. On first thought, no one would suspect that removing a major road can improve traffic flow — yet that’s exactly what it does (or would do) in some cases. The flipside of this contrarian coin is that building a brand new highway often fails to alleviate the congestion that inspired its construction in the first place. Economists Gilles Duranton and Matthew Turner of the University of Toronto offer an impressive and empirical explanation for this concept in an upcoming issue of the American Economic Review (full paper via Google Docs).

Duranton and Turner analyzed loads of data on traffic, infrastructure, and travel behavior from metropolitan regions across the United States and found that “vehicle-kilometers traveled … increases proportionately to roadway lane kilometers for interstate highways.” For those who don’t care for either academic abstracts or the metric system, the authors then parse their conclusion in pithier terms: “roads cause traffic.” The basis for this confusing reality, write Duranton and Turner, is a three-pronged “fundamental law of highway congestion” that explains why road construction can never keep pace with road congestion:

people drive more when the stock of roads in their city increases; commercial driving and trucking increase with a city’s stock of roads; people migrate to cities which are relatively well provided with roads.

As compelling as the evidence in this study may be, it doesn’t tell perceptive urban planners anything they haven’t known for decades. Duranton and Turner trace the “fundamental law of highway congestion” to a study published by road researcher Anthony Downs in 1962. In an issue of Traffic Quarterly, Downs reported his law of “peak-hour” congestion: “This Law states that on urban commuter expressways, peak-hour traffic congestion rises to meet maximum capacity.”

Before Downs, social critic Lewis Mumford recognized the properties of this law in the behavior of drivers in and around New York City. Mumford was well ahead of his contemporaries when it came to recognizing the perils of road construction; Jane Jacobs is often remembered as the foil of road-builder Robert Moses, but at the time many would have cast Mumford in that role. While researching my history of transportation in the Northeast, I came across an illuminating four-part series Mumford wrote for the New Yorker in 1955 called “The Roaring Traffic’s Boom.” In the second article, Mumford provides a commentary that anticipates Duranton and Turner half a century ago:

[O]ur one-eyed specialists continue to concoct grandiose plans for highway development, as if motor transportation existed in a social vacuum. … Instead of curing congestion, they widen chaos. …

All the current plans for dealing with congestion are based on the assumption that it is a matter of highway engineering, not of comprehensive city and regional planning, and that the private motorcar has priority over every other means of transportation, no matter how expensive it is in comparison with public transportation, or how devastating its by-products.

Prescient as he was, Mumford had little impact on the policies of his day, partly because his suggested solutions — carefully planned communities, population limits, and a fully balanced transportation network — were even more objectionable than curtailing road construction. (And you thought raising the gas tax was politically unpalatable.)

Likewise the findings of Duranton and Turner are the type that should, but probably will not, inform good social policy. This is not a blatant call for more public transportation funding; in fact, the Toronto researchers found that adding transit does nothing to ease highway congestion. When one driver leaves the road, another simply takes his or her place, as Turner explained to Streetsblog DC. (That finding will be misappropriated by road-building enthusiasts to argue for more highways under the fundamental “if nothing helps, then who cares?” error.)

The only solution endorsed by Duranton and Turner is congestion pricing. In April 1955, Mumford predicted that Manhattan would soon have to “banish private wheeled traffic” in midtown during the day. This extreme version of congestion control did not come soon, and when Mayor Bloomberg did introduce a pricing plan recently, it was roundly beaten. “People, it seems, find it hard to believe that the cure for congestion is not more facilities for congestion,” Mumford wrote. Some beliefs are hard to change.

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Entry filed under: Planning, Traffic, Transit, Trasportation. Tags: , , , , , , , , , , , , .

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