The calculator is limited to use for capacity expansions (lane additions, roadway lengthening, and new facility construction). Caltrans’ recommendation is supported by the report of a panel of experts convened by Caltrans to guide its choice of induced travel estimation methods for the TAF ( Deakin et al., 2020).Ĭalculator use is not appropriate in all circumstances or for all purposes, however. That corresponds to interstate highways (class 1), other freeways and expressways (class 2), and other principal arterials (class 3).Ĭaltrans now recommends in its Transportation Analysis Framework (TAF) that the calculator be used where possible to estimate – or at least benchmark – induced VMT: “In cases where the NCST Calculator can be directly used, it should either be used exclusively or used to benchmark results from a ” ( Caltrans, 2020, p. The calculator applies only to publicly owned facilities with Federal Highway Administration (FHWA) functional classifications of 1, 2 or 3. This calculator allows users to estimate the VMT induced annually as a result of expanding the capacity of publicly owned roadways, like those managed by the California Department of Transportation (Caltrans), in one of California’s urbanized counties (counties within a metropolitan statistical area (MSA)). Please also see the References section below for a list of relevant studies. For a summary of key studies estimating the elasticity of VMT with respect to lane miles, see Handy and Boarnet ( 2014b). These longer-term elasticities account for short-run shifts in travel (as people take advantage of the increased capacity and travel speed by driving more), as well as longer-run dispersion of residential and business location and development.įor more information on the induced travel and other impacts of highway expansion, see Handy ( 2015) and Handy and Boarnet ( 2014a). interstates) than for capacity increases on other roadways. In general, the studies show that a 10-percent increase in roadway capacity is likely to increase network-wide VMT by 6 to 10 percent (an elasticity of 0.6 to 1.0) in the long run (5 to 10 years), with greater elasticities for expansions of major highways (e.g. 
Most recent studies have estimated elasticities in the same ballpark, despite using a range of methods to control for other VMT-inducing factors and the bi-directional relationship between VMT and capacity expansion. An elasticity of 1.0 indicates that a given percent increase in lane miles will cause the same percent increase in VMT. The greater the elasticity, the greater the increase in VMT from a given increase in roadway capacity. The magnitude of the induced travel effect is commonly measured as the elasticity of VMT with respect to lane miles: Indeed, it is explained by the bedrock economic principles of supply and demand: “adding capacity decreases travel time, in effect lowering the ‘price’ of driving when prices go down, the quantity of driving goes up” ( Handy and Boarnet, 2014a). The induced travel concept is not new (see the “fundamental law of highway congestion” Anthony Downs suggested in 1962). That increase in VMT is called “induced travel.” Miles traveled (VMT) by a nearly equivalent proportion within a few years, reducing or They consistently show thatĪdding roadway capacity in congested areas actually increases network-wide vehicle But studiesĮxamining that approach indicate it is only a temporary fix. by building new roadways or adding lanes to existing facilities. Attempts to address traffic congestion commonly rely on increasing roadway capacity,Į.g.
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