Traditional evaluation of the transportation system focuses on automobile traffic flow and congestion reduction. However, this paradigm is shifting. In an effort to combat global warming and reduce greenhouse gas (GHG) emissions, a number of cities, regions, and states across the United States have begun to deemphasize vehicle delay metrics such as automobile Level of Service (LOS). In their place, policymakers are considering alternative transportation impact metrics that more closely approximate the true environmental impacts of driving. One metric increasingly coming into use is the total amount of driving or Vehicle Miles Traveled (VMT). While state goals for reducing GHG emissions have been one motivation for the shift to VMT measures, reductions in VMT produce many other potential benefits, referred to as “cobenefits,” such as reductions in other air pollutant emissions, water pollution, wildlife mortality, and traffic congestion, as well as improvements in safety and health, and savings in public and private costs. Such benefits may provide additional justification for reducing VMT. In this paper, the authors review the literature to explore the presence and magnitude of potential co-benefits of reducing VMT, providing California-specific examples where available.

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Expanding roadway capacity often leads to commensurate increases vehicle miles traveled (VMT). This is the “induced travel” effect—a net increase in VMT across the roadway network due to an increase in roadway capacity. This increase in VMT erodes any initial reduction in congestion and causes increased greenhouse gas and local air pollutant emissions. Yet highway expansion projects continue to be proposed across the US, often using congestion relief—and sometimes greenhouse gas reductions— as a justification for adding lanes. The existence of these rosy projections about highway expansion projects indicates that the induced travel effect is often not fully accounted for in travel demand models or in the environmental review process for the projects, as prior research has shown.1 With these problems in mind, researchers at the University of California, Davis developed and launched an online tool in 2019—the NCST Induced Travel Calculator—to help agencies estimate the VMT induced annually by adding lanes to major roadways in California’s urbanized counties. With Calculator use increasing, the UC Davis researchers initiated a project to update the Calculator and improve its functionality based on recent data and empirical research.

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Accessory dwelling units (ADUs) are small, self-contained housing units that share the same lot as a primary dwelling, usually a single-family detached house. In places with major housing shortages ADUs can be an efficient and cost-effective way to increase supply. Over the last few years, the California Legislature has passed laws to reduce barriers to permitting ADUs, and some California cities have liberalized their regulations even further. However, loosening ADU regulations—particularly those related to parking—can spark neighborhood opposition to ADUs. Recent surveys indicate that both homeowners and local government staff remain concerned that ADUs will overwhelm neighborhood parking supplies.

Whether these concerns are justified is unclear. A UC Davis-led research team surveyed 396 homeowners in Sacramento and collected lot size and other data to investigate whether the total effective parking supply of the average single-family detached home is sufficient to accommodate the vehicles associated with the residents of both a primary dwelling and a potential ADU. This policy brief summarizes the findings from that research and provides policy implications.

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Building additional roadway capacity—via constructing entirely new roadways or extending or adding lanes to existing roadways—is often proposed as a solution to traffic congestion and even as a way to reduce greenhouse gas (GHG) emissions. The logic for the latter is that increasing roadway capacity increases average vehicle speeds, which improves vehicle fuel efficiency and reduces per-mile emissions of GHGs and local air pollutants. But that logic relies on the flawed assumption that the amount that people drive does not change when the time it takes to drive places changes. In fact, the amount that people drive does respond to changes in driving times. Empirical research demonstrates that as roadway supply increases, vehicle miles traveled (VMT) generally does, too. This is the “induced travel” effect—a net increase in VMT across the roadway network due to an increase in roadway capacity, which ultimately erodes any initial increases in travel speeds and causes increased GHG emissions. Researchers at the University of California, Davis reviewed the empirical research on induced travel to understand the likely effects of adding roadway capacity in a variety of contexts.

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Changes are coming to housing development and demand in California. The state’s sprawling development patterns have come under increasing scrutiny as the state struggles to reduce its greenhouse gas emissions, abate a decades-long housing supply and affordability crisis, and meet the needs of the largest generation in American history – the millennials (Generation Y). In this dissertation, I explore three ways in which residential development and demand in California could change going forward.

In my first study (Chapter 2 of this dissertation), I investigate how an upcoming change in California’s project-level environmental review law (the California Environmental Quality Act or CEQA) could affect the approval process for urban development. The state recently mandated that local, regional, and state agencies must replace “level of service” (LOS) with vehicle miles traveled (VMT) as the primary measure – and basis for mitigation – of transportation impacts under CEQA by July 1, 2020. I use a historical counterfactual approach to assess how replacing LOS with VMT could have impacted the approval process for 153 land development projects over 16 years in the City of Los Angeles. I find that most projects could have qualified for at least some environmental review streamlining under the VMT-based framework recommended by the state, including over 75 percent of residential-containing projects. My results suggest that swapping LOS for VMT could reduce the environmental review burden for development in urban areas and provide some of the approval process streamlining necessary to increase housing production in California. And because the streamlined development would be in areas characterized by lower VMT per capita than the regional average, it would likely contribute to reducing VMT per capita in line with state targets. 

In my second study (Chapter 3 of this dissertation), I look at accessory dwelling units (ADUs). How much ADUs can help with California’s housing supply and affordability crises depends on the homeowners who do not yet own one – their willingness and ability to build an ADU will determine the ceiling for ADU construction. I use a survey of 502 single-family homeowners in the Sacramento metropolitan area to investigate homeowners’ willingness to consider building an ADU, and the motivations and barriers they face. I find that as many as 54.1% of Sacramento city single-family detached homeowners could either have an ADU or be open to creating one. Familiarity with ADUs has the strongest association with openness to building an ADU in my logistic regression model. And homeowners’ top-ranked motivation for creating an ADU is housing family or friends. Cost-related concerns ranked as the biggest obstacles to creating an ADU, followed by permitting and regulatory issues. My findings suggest that ADUs have significant potential to help California close its housing supply gap.

In my third study (Chapter 4 of this dissertation), I explore how millennials – people born between 1982 and 2000 – choose where to live. Surveys suggest that millennials have a stronger preference than previous generations for urban amenities. But studies also indicate that most millennials will eventually settle in a suburb. That raises big questions for urban planners and policymakers, as well as for the future of sustainable urbanism. If most millennials will end up suburbanizing, what happens to their erstwhile preferences for urban amenities? Do they seek out suburban neighborhoods with urban amenities? Do their preferences simply change with time and major life events? I use in-depth interviews of 20 households who recently purchased homes in the San Francisco Bay Area to explore how millennials choose where to live when they reach the life cycle stages typically associated with bigger homes in suburban areas. I find that life cycle effects emerged in different ways for the households I interviewed. As they partnered and began having or thinking about having children, most households suburbanized or planned to suburbanize in the future. The households still valued urban amenities, but they generally did not prioritize urban amenities when searching for their suburban homes, with one exception – proximity to commuter transit.

Local governments have long relied on Level of Service (LOS), a measure of automobile congestion, as the basis for assessing transportation impacts of land use development projects. However, use of the LOS metric creates an incentive for projects that contribute to urban sprawl while penalizing denser development projects that could allow people better accessibility to jobs and services through alternate modes like walking, bicycling, or transit. Starting July 1, 2020, local governments in California are required to use vehicle miles traveled (VMT) rather than LOS to measure land use projects’ transportation impacts. Researchers at UC Davis studied how this change affects the approval process for urban development. Because most agencies have not yet switched to using VMT in their analyses, the researchers looked back at environmental documents for development projects in the City of Los Angeles between 2001 and 2016 and determined whether these projects could have benefited from using a VMT metric instead of LOS for measuring their transportation impacts. Findings are summarized in this policy brief.

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The National Center for Sustainable Transportation’s (NCST’s) Induced Travel Calculator (Calculator) has generated substantial interest among policymakers and practitioners as a method for estimating induced vehicle miles traveled (VMT). With Calculator use increasing, the Institute of Transportation Studies at University of California, Davis (ITS-Davis) initiated a project to update the Calculator and improve its functionality based on recent data and empirical research. Efforts included adding three more years of baseline VMT and lane mile data to the Calculator (2017, 2018, and 2019), adding ranges to the Calculator’s induced VMT estimates (+/-20%), and providing an updated review of the induced travel literature. The team also investigated and determined that there is not enough empirical evidence to justify using different elasticities based on initial congestion levels, urban versus rural setting, or lane type (for general-purpose lanes, high-occupancy-vehicle lanes, and high-occupancy toll lanes). Going forward, this report suggests avenues for future induced travel research, including meta-analyses of induced travel studies to estimate pooled effect sizes, more research on the impact of existing traffic congestion and other contextual factors on induced travel effect size, and further studies on induced travel from managed lanes. It will also be important to continue monitoring other induced travel calculators for consistency with NCST’s Calculator.

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This white paper presents the results of a survey administered by the University of California Pavement Research Center (UCPRC) exploring the successes, challenges, funding, and organizational structure of six centers in other states that share a similar mission to support the improvement of city and county pavement practices. Five of the six centers that participated in the survey are statewide centers located in Iowa, Minnesota, North Dakota, Ohio and Texas. The sixth is a regional center located in Washoe County, Nevada, the Regional Transportation Commission. These centers were selected as being the nation’s most advanced based on an extensive internet search and discussions with key pavement professionals across the country.

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Senate Bill (SB) 743 (2013) and its related regulations eliminated automobile level of service (LOS) and replaced it with vehicle miles traveled (VMT) as the primary transportation impact metric for land development projects under the California Environmental Quality Act. Actual implementation of the LOS-to-VMT shift was left up to lead agencies, primarily local governments. The LOS-to-VMT shift was expected to create many challenges, given the often-limited resources of local governments, the entrenched use of LOS, and the perceived lack of established practice regarding VMT estimation, mitigation, and monitoring. With those concerns in mind, researchers at the University of California, Davis investigated how local governments have been implementing the LOS-to-VMT shift for land development projects.  This policy brief summarizes the findings from that investigation.

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Induced travel is a well-documented effect in which expanding highway capacity increases the average travel speed on the highway, which in turn reduces the perceived “cost” of driving and thereby induces more driving. This increase in vehicle miles traveled (VMT) increases congestion (often back to pre-expansion levels) and air pollutant emissions, reducing or eliminating the purported benefits of the expansion. Yet highway expansion projects continue to be proposed across California, often using congestion relief—and sometimes greenhouse gas reductions—as a justification for adding lanes. These rosy projections about the benefits of highway expansion projects indicate that the induced travel effect is often not fully accounted for in travel demand models or in the projects’ environmental review process.

With this problem in mind, researchers at the University of California, Davis developed an online tool to help agencies estimate the VMT induced annually by adding lanes to major roadways in California’s urbanized counties. The researchers also applied the calculator to estimate the vehicle travel induced by five highway expansion projects in California that had gone through environmental review within the past 12 years. They then compared their estimates with the induced travel analysis completed for the projects’ actual environmental impact assessments. This policy brief summarizes findings from that research, along with policy implications.

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