![]() ![]() Third, since most metro areas in recent decades have added relatively little freeway capacity, the authors’ analytical results, even if correct, would tell us only that making marginal increases in freeway capacity produces little in the way of freeway congestion reduction. In 2014, Randal O’Toole’s Antiplanner blog provided data on daily VMT per lane-mile on urban freeways around the country, finding a range from as low as 9,000 to a high of 22,000. And the extent of congestion also varies enormously among large metro areas overall. But in fact, actual congestion levels (in both extent and duration) vary considerably among the set of a metro area’s freeways. Second, the authors’ premise that all such freeways have a “natural level of saturation” implies that all urban Interstates should fill up to the point of serious peak-period congestion. That leaves them unable to analyze (as opposed to only speculating about) the extent to which drivers shift trips from parallel arterials to the newly expanded urban Interstate (which would not represent new driving, but simply a re-allocation of existing driving). Analytically, they treat parallel arterials and all other roadways in the metro area as a large blob. First, their detailed analysis deals only with urban Interstates. Here are some of the limitations with what Duranton and Turner did. In effect, they claimed, highway supply creates its own demand.īut some people opposed to adding capacity to highways are claiming stronger results than Duranton and Turner’s analysis produced-a purported one-for-one correlation (always and everywhere) between highway capacity growth and VMT growth. Here is what they did: Using federal highway data plus data from the National Household Travel Survey, their econometric analysis found evidence that vehicle miles of travel (VMT) on urban Interstates increases proportionally to highway capacity. Economists Gilles Duranton and Matthew Turner titled the paper “ The Fundamental Law of Road Congestion: Evidence from U.S. The paper appeared in the American Economic Review in 2011. In my book, Rethinking America’s Highways (University of Chicago Press, 2018), I analyzed that academic paper and found that what the researchers did prove was actually far less than highway opponents claim. They increasingly cite an academic study, which they claim proved this theory, and have taken to calling it the “iron law of roadway congestion.” Many of those who oppose adding lanes to congested freeways argue that doing this is futile because the new lanes quickly fill up with cars and traffic congestion is soon back to what it was before.
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