Eric Jaffe

Building Roads Creates Traffic

Writer, The Infrastructurist Blog, Jun 10, 11.

Tagged with transportation, infrastructure, congestion, vehicle miles traveled, traffic

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.”

Full Story: Why building roads creates traffic
Source: The Infrastructurist: America Under Construction Blog, Jun 10, 2011

More Commentary