SCAG Expert Panel on Induced Travel Impacts of Priced Managed Lanes
The SCAG-funded expert panel study “Induced Travel Estimation Revisited,” authored by Professor Michael Manville and published this week by the Institute of Transportation Studies at UCLA is an early examination of how changing environmental regulations apply to new express lanes, also known as priced managed lanes. The report specifically examines express lanes’ effect on vehicle miles traveled (VMT), an important but complicated question in navigating relatively recently adopted policies using it as a measure of environmental impact. SCAG convened the panel to investigate the issue relevant to many regional transportation projects currently in the pipeline and further its work to monitor and evaluate transportation investments’ effectiveness in reaching California’s climate and equity goals. This panel included the following experts whom Professor Manville consulted and whose input was considered in his conclusions: Gilles Duranton of the University of Pennsylvania; Bruce Griesenbeck of Fehr & Peers; Susan Handy of University of California, Davis; Donald Hubbard of GHD; Chris McCahill of the State Smart Transportation Initiative; Ronald Milam of Fehr & Peers; Matthias Sweet of Toronto Metropolitan University, and Brian Taylor of UCLA.
Senate Bill (SB) 743 changed the way California evaluates new transportation projects under the California Environmental Quality Act (CEQA), replacing traffic level of service (automobile delay) with vehicle miles traveled (VMT) as a metric of significant environmental impact, essentially treating VMT as a pollutant to be analyzed and mitigated. Much of the existing research on VMT increases resulting from new roadway capacity, or induced demand, does not distinguish between the addition of general-purpose lanes and priced managed lanes. To help make a distinction, SCAG worked with Caltrans to seek expert advice on how SCAG, its regional partners, and the state should approach compliance with SB 743 implementation for projects that add priced managed lane capacity. This question is critical because most of the region’s proposed new highway capacity projects are priced managed lanes.
In 2020, Caltrans assembled and asked an expert panel to review approaches for estimating induced travel, the concept that adding roadway capacity increases VMT by making more space available for traffic to fill. A portion of this effort looked at special purpose lanes, including express lanes, and concluded more study was required to establish a strong evidentiary basis for estimating induced travel effects. Subsequently, SCAG convened another expert panel chaired by Professor Manville of the Luskin School of Public Affairs. Professor Manville does not counter the earlier panel’s conclusion but indicates that in the absence of new empirical data, it is likely that new express lanes would induce new travel at the regional scale, like any new freeway capacity.
The study report recognizes that express lanes are different—for example, they carry higher value trips, less congestion, and probably less pollution—but these characteristics are specific to the express lane corridor. However, the state is concerned with how a newly built facility changes regional VMT, meaning not just the new driving that occurs on the express lane but also new driving on other, existing roads because the new express lane was built. The state believes the main reason for the impact on regional VMT is that land use will change in response to improved mobility.
Induced demand, Professor Manville writes in the study report, would occur when a new express lane is built and traffic redistributes on that new lane, opening space on existing unpriced lanes and allowing other drivers to backfill and add VMT. The study report also notes that induced demand also happens when rail service is added, although it is clarified that a train is unlikely to increase VMT—but it doesn’t reduce VMT either; this concept also casts doubt on transit and most mitigation measures as effective mitigation strategies.
As Professor Manville states, no empirical research specifically looks at how priced managed lanes induce travel at the regional level. He suggests that further research could replicate recent induced travel studies and seek to separate data on VMT increases attributed to new general-purpose lane capacity versus new priced managed lane capacity. SCAG sees value in pursuing further study to evaluate the regional elasticity, or the measure of responsiveness to variables, for priced managed lanes using more recent data.
Though not the central focus of the study report, perhaps the most thought-provoking takeaway from Professor Manville’s assessment is that VMT is not pollution; it has mobility and economic benefits as well as environmental and congestion costs, and the overall social value of a given vehicle mile traveled can vary widely depending on when and where the travel occurs. This raises larger questions about state policies’ reliance on regulating VMT as a proxy for environmental impact rather than targeting pollution or congestion directly. SCAG understands this question does not justify building more roads, but it does have implications for SB 743 implementation, as well as SB 375, where VMT is treated as a proxy for transportation carbon emissions. As the relationship between VMT and transportation carbon emissions (and criteria pollutants) weaken due to electrification and clean technology implementation, SCAG believes these are important questions to explore.
SCAG concurs that other metrics of measurement can be helpful in understanding the benefits, costs, and performance of our transportation system – which are complex and nuanced. As of now, state policies are focused on VMT at the expense of evaluating transportation holistically, leaving gaps in the ability to evaluate system performance as our technology landscape and understanding of transportation’s value evolve. Such explorations can aid transportation planners in optimizing system performance to align with policy goals and guide effective investments.