The Calculator can be used to estimate induced VMT from additions of general-purpose lanes, HOV lanes, and HOT lanes.However, our best estimate based on the empirical research is that the effects of capacity reductions are symmetric with the effects of capacity increases. It cannot be used to estimate VMT effects of capacity reductions or lane type conversions. The Calculator is limited to use for capacity expansions (lane additions, roadway lengthening, and new facility construction).The following caveats apply, which are discussed further on the FAQ page: 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 might 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 included the Calculator in its 2020 Transportation Analysis Framework (TAF) as a method 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 currently applies only to publicly owned facilities with 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, reviews, and policy briefs. These longer-term elasticities account for shorter-term shifts in travel (as people take advantage of the increased capacity and reduced cost of vehicle travel by driving more), as well as longer-run growth and dispersion of residential and business location and development.įor more information on the induced travel effect and a review of the empirical studies that have explored the phenomenon and estimated induced travel elasticities, see Volker and Handy ( 2022). In general, the studies show that a 10-percent increase in capacity is likely to increase network-wide VMT by at least 7 to 10 percent (an elasticity of 0.7 to 1.0) in the long run (3 to 10 years), at least for capacity expansions on major roadways with Federal Highway Administration (FWA) functional classifications of 1, 2, or 3. Most recent studies have estimated elasticities in the same ballpark, despite using a range of methods to control for other VMT-inducing factors (such as population, income, geography, and fuel cost) and the bi-directional relationship between VMT and capacity expansion. The greater the elasticity, the greater the increase in VMT from a given increase in roadway capacity. An elasticity of 1.0 means that VMT will increase by the same percentage as the increase in lane miles. The elasticity is the percentage increase in VMT in the studied area that results from a 1% increase in lane miles in that area. The magnitude of the induced travel effect is commonly measured as the elasticity of VMT with respect to lane miles: And 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 phenomenon has been theorized and anecdotally observed for more than a century ( 1962, 1992, 2004 Ladd, 2013). That increase in VMT is called “induced travel.” They consistently show that adding roadway capacity actually increases network-wide vehicle miles traveled (VMT) by a nearly equivalent proportion within a few years, reducing or negating any initial congestion relief. But studies examining 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, e.g.
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