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PO <br />L <br />I <br />C <br />Y <br /> <br />BR <br />I <br />E <br />F <br />October 2015 <br />Increasing Highway Capacity Unlikely <br />to Relieve Traffic Congestion <br />Reducing traffic congestion is often <br />proposed as a solution for improving fuel <br />efficiency and reducing greenhouse gas <br />(GHG) emissions. Traffic congestion has <br />traditionally been addressed by adding <br />additional roadway capacity via constructing <br />entirely new roadways, adding additional <br />lanes to existing roadways, or upgrading <br />existing highways to controlled-access <br />freeways. Numerous studies have examined <br />the effectiveness of this approach and <br />consistently show that adding capacity to <br />roadways fails to alleviate congestion for <br />long because it actually increases vehicle <br />miles traveled (VMT). <br />An increase in VMT attributable to increases <br />in roadway capacity where congestion <br />is present is called “induced travel”. The <br />basic economic principles of supply and <br />demand explain this phenomenon: adding <br />capacity decreases travel time, in effect <br />lowering the “price” of driving; and when <br />prices go down, the quantity of driving <br />goes up.1 Induced travel counteracts the <br />effectiveness of capacity expansion as a <br />strategy for alleviating traffic congestion and <br />offsets in part or in whole reductions in GHG <br />emissions that would result from reduced <br />congestion. <br />Susan Handy <br />Department of Environmental Science and Policy <br />University of California, Davis <br />National Center for Sustainable Transportation • 1 <br />Issue <br />Contact Information: <br />slhandy@ucdavis.edu <br />Increased roadway capacity induces <br />additional VMT in the short-run and even <br />more VMT in the long-run. A capacity <br />expansion of 10% is likely to increase VMT <br />by 3% to 6% in the short-run and 6% to <br />10% in the long-run. Increased capacity <br />can lead to increased VMT in the short-run <br />in several ways: if people shift from other <br />modes to driving, if drivers make longer <br />trips (by choosing longer routes and/or <br />more distant destinations), or if drivers <br />make more frequent trips.3,4,5 Longer-term <br />effects may also occur if households and <br />businesses move to more distant locations <br />or if development patterns become more <br />dispersed in response to the capacity <br />increase. One study concludes that the <br />full impact of capacity expansion on VMT <br />materializes within five years6 and another <br />concludes that the full effect takes as long as <br />10 years.7 <br />Capacity expansion leads to a net increase <br />in VMT, not simply a shifting of VMT from <br />one road to another. Some argue that <br />increased capacity does not generate new <br />VMT but rather that drivers simply shift from <br />slower and more congested roads to the new <br />or newly expanded roadway. Evidence does <br />not support this argument. One study found <br />“no conclusive evidence that increases in <br />state highway lane-miles have affected traffic <br />on other roads”8 while a more recent study <br />concluded that “increasing lane kilometers <br />for one type of road diverts little traffic from <br />other types of roads”.9 <br />Increases in GHG emissions attributable <br />to capacity expansion are substantial. One <br />study predicted that the growth in VMT <br />attributable to increased lane miles would <br />produce an additional 43 million metric tons <br />of CO2 emissions in 2012 nationwide.10 <br />Key Research Findings <br />The quality of the evidence linking highway <br />capacity expansion to increased VMT <br />is high. All studies reviewed used time- <br />series data and sophisticated econometric <br />techniques to estimate the effect of <br />increased capacity on congestion and <br />VMT. All studies also controlled for other <br />factors that might also affect VMT, including <br />population growth, increases in income, <br />other demographic factors, and changes in <br />transit service.2