Attempts to address traffic congestion commonly rely on increasing roadway capacity, e.g. by building new roadways or adding lanes to existing facilities. But studies examining that approach indicate it is only a temporary fix. They consistently show that adding roadway capacity in congested areas actually increases network-wide vehicle miles traveled (VMT) by a nearly equivalent proportion within a few years, reducing or negating the initial congestion relief. That increase in VMT is called “induced travel.”
The induced travel concept is not new (see the “fundamental law of highway congestion” Anthony Downs suggested in 1962). 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 magnitude of the induced travel effect is commonly measured as the elasticity of VMT with respect to lane miles:
The greater the elasticity, the greater the increase in VMT from a given increase in roadway capacity. An elasticity of 1.0 indicates that a given percent increase in lane miles will cause the same percent increase in VMT.
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. 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. interstates) than for capacity increases on other roadways. 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.
For more information on the induced travel and other impacts of highway expansion, see Handy (2015) and Handy and Boarnet (2014a). For a summary of key studies estimating the elasticity of VMT with respect to lane miles, see Handy and Boarnet (2014b). Please also see the References section below for a list of relevant studies.
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)). The calculator applies only to publicly owned facilities with Federal Highway Administration (FHWA) functional classifications of 1, 2 or 3. That corresponds to interstate highways (class 1), other freeways and expressways (class 2), and other principal arterials (class 3).
Caltrans 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 [travel demand model]” (Caltrans, 2020 , p. 14). 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).Calculator use is not appropriate in all circumstances or for all purposes, however. The following caveats apply:
To estimate induced VMT for user-specified capacity expansion projects, the calculator simply solves this equation:
Data sources and specifications for the equation inputs are as follows:
Lane Miles. The calculator uses lane mileage data from Caltrans’ Transportation System Network (TSN) database (similarly reported in the Highway Performance Monitoring System [HPMS]). The user has the option to use baseline data from 2016, 2017, 2018, or 2019. The percent change in lane miles is calculated by dividing the number of project-added lane miles (input by the user) by the total lane miles of the same facility type in the same geography. For interstate highways (FHWA functional class 1), lane mileage is calculated at the MSA level. For other freeways, expressways and major arterials (classes 2 and 3) managed by public agencies, lane mileage is calculated at the county level. The choice of geographies is discussed further below, in conjunction with elasticities. The data is available here.
Existing VMT. The calculator uses VMT data retrieved using Caltrans’ TSN and HPMS database. The user has the option to use baseline data from 2016, 2017, 2018, or 2019. The VMT is tallied for each county and each FHWA functional classification. Existing VMT on interstate highways (FHWA functional class 1) is calculated at the MSA level, and existing VMT on other publicly owned freeways, expressways and major arterials (classes 2 and 3) is calculated at the county level. The data is available here.
Elasticities. The calculator uses an elasticity of 1.0 for lane additions to interstate highways, and an elasticity of 0.75 for lane additions to class 2 or 3 facilities.
For interstate highways, the 1.0 elasticity derives from Duranton and Turner (2011) -- perhaps the most thorough and stringently vetted induced travel study to date – as well as subsequent studies that also use robust statistical methods to address the simultaneity bias (the fact that increasing VMT an spur roadway expansion in addition to being caused by it). Duranton and Turner’s study used data from 1983, 1993 and 2003 for all MSAs in the United States that had nonzero interstate lane mileage in all three years. Among other modeling, the study used a two-stage least squares regression with three instrumental variables to estimate the elasticity of vehicle kilometers traveled (VKT) on interstate highways in the 228 studied MSAs with respect to interstate lane kilometers in those MSAs. The authors concluded that their estimation method better controls for the possible bi-directional relationship between VMT and lane miles than the methods used in previous studies. Using that “preferred estimation method,” their “preferred estimate” was a long-run (10-year) elasticity of 1.03. However, while the authors concluded that “diversion of traffic from other road networks does not appear to play a large role,” they cautioned that they could not “rule out the absence of a substitution effect.” They estimated that the “diversion of traffic from other classes of roads accounts for between 0 and 10 percent of the total [induced] interstate VKT.”
Like Duranton and Turner (2011), the calculator uses MSAs as the unit of analysis for interstate highway capacity expansions. The calculator also uses a similar VMT elasticity (1.0), albeit rounded down (in part to account for the (small) potential substitution effect). That accords with Handy and Boarnet’s (2014a) conclusion that the best estimate for the long-run VMT elasticity for highway lane additions is close to 1.0. It is also consistent with more recent studies that have estimated long-run induced VMT elasticities using instrumental variables, including Melo et al. (2012) (elasticity of 0.989 for major [class 3] and minor [class 4] arterials in 100 urbanized areas in the U.S.), Graham et al. (2014) (elasticity of 0.772 for class 1, 2, 3, and 4 [minor arterial] facilities in 101 urbanized areas in the U.S.), Hsu and Zhang (2014) (elasticity of between 1.24 and 1.34 for Japan’s national expressways), and Hymel (2019) (elasticity of between 0.892 and 1.063 for freeways and other limited-access roads in Census-defined urban areas in the U.S.). See Volker et al. (2020) for further discussion of the induced travel literature and the analysis of induced travel in the environmental review process.
For other highways, expressways and major arterials managed by public agencies, the 0.75 elasticity derives from Duranton and Turner (2011) and Cervero and Hansen (2002), as well as subsequent studies that also use robust statistical methods to address the simultaneity bias (the fact that increasing VMT an spur roadway expansion in addition to being caused by it). While Duranton and Turner (2011) could not use their preferred method to estimate elasticities for state highways and other “major roads” besides interstates, their elasticity estimates using ordinary least squares regression all fall between 0.67 and 0.89. Cervero and Hansen (2002) similarly estimated an intermediate-run (5-year) VMT elasticity of 0.79 for lane mile additions to state-owned roadways in California’s urbanized counties (then 34, now 37), using three-stage least squares regression. Those elasticities are similar to other longer-term elasticities calculated for combined major road types (not just interstate highways) in California and across the United States (Handy and Boarnet, 2014a).
Like Cervero and Hansen (2002), the calculator uses urbanized counties (those within MSAs) as the unit of analysis for capacity expansions on non-interstate highways, expressways and major arterials managed by public agencies. The calculator also uses a similar VMT elasticity for those facilities (0.75) as Cervero and Hansen (2002) estimated for state-owned roadways (0.79), and within the range Duranton and Turner (2011) found for non-interstate “major roads” in the urbanized areas of MSAs (0.67 to 0.89). The calculator’s 0.75 elasticity is rounded down from the estimates of Cervero and Hansen (2002) and Duranton and Turner (2011), in part to account for the (small) potential substitution effect discussed in Duranton and Turner (2011). The 0.75 elasticity is also similar to the longer-term elasticities calculated in other recent studies for combined major road types (not just interstate highways) in the U.S. (Hymel, 2019; Handy & Boarnet, 2014a; Graham et al., 2014; Melo et al., 2012). See Volker et al. (2020) for further discussion of the induced travel literature and the analysis of induced travel in the environmental review process.
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