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Open Access Publications from the University of California

The primary mission of the Institute is research - cross-disciplinary inquiries into emerging transportation issues with great societal significance. It draws upon campus researchers and graduate students from a variety of disciplines, and also upon other universities and research centers around the world.

Cover page of Barcodes, virtual money, and Golden Wheels: The influence of Davis, CA schools' bicycling encouragement programs

Barcodes, virtual money, and Golden Wheels: The influence of Davis, CA schools' bicycling encouragement programs


Efforts to encourage bicycling to school can achieve numerous societal benefits, including improved childhood health, reduced traffic congestion, and even long-term effects such as increased bicycling skill and attitudes. Most of the literature on children bicycling to school focuses on the influence of infrastructure interventions, yet relatively few studies have robustly evaluated the influence of encouragement efforts. This study seeks to examine the effects of three encouragement efforts undertaken at primary and secondary schools in Davis, California: the scanning program, the Monkey Money incentive system, and the national Bike-to-School Day celebration. I use a binomial regression to statistically analyze bicycle rack count data and Safe Routes to School classroom tallies collected by city employees and local volunteers. After accounting for the schools’ physical environment and characteristics, as well as the influence of weather and the natural environment, I find that all three of the encouragement efforts increase levels of bicycling to school. I conclude by suggesting that these encouragement programs have the potential for lasting influence by providing children with the skills and confidence to bicycle later in life. I also note the value of further state support for the parent volunteers who operate these encouragement programs, in order to allow the spread of similar encouragement programs across a variety of cities, including disadvantaged communities.

Cover page of Do bicycling experiences and exposure influence bicycling skills and attitudes? Evidence from a bicycle-friendly university

Do bicycling experiences and exposure influence bicycling skills and attitudes? Evidence from a bicycle-friendly university


Life changes are often associated with changes in travel behavior, due to a break in habitual travel cues and the introduction of a novel travel context. Universities provide a particularly appropriate setting to examine how these life changes can bring about changes in travel attitudes, 27 norms, and skills – which together form a psychological construct called “motility” that describes the capability for travel. In this study, I pool data from seven years of the University of California, Davis’ annual campus travel survey to create a longitudinal panel, and use a retrospective survey to collect the bicycling behaviors, attitudes, and skills of undergraduates every year since they graduated from high school. I find that, on average, UCD undergraduates’ pro-bicycling attitudes decrease slightly over time while bicycling skills increase substantially throughout college. I then use the retrospective panel data to estimate a statistical model to analyze the influence of bicycling exposure and experiences on skills and attitudes. I find that riding a bicycle at any point during college increases both pro-bicycling attitudes and bicycling skills, while exposure to high levels of bicycling appears not to influence attitudes or skills. This study provides confirmatory evidence for the motility approach and suggests possible policy avenues, such as incentivizing short-term bicycle use in order to shift perceptions and attitudes about bicycling, with the intent of fostering a positive feedback cycle between greater bicycling attitudes and skills and increased bicycle use.

Cover page of Understanding the Impact of Reoccurring and Non-Financial Incentives on Plug-in Electric Vehicle Adoption – A Review

Understanding the Impact of Reoccurring and Non-Financial Incentives on Plug-in Electric Vehicle Adoption – A Review


The market introduction of plug-in electric vehicles (PEVs) is being partially driven by policy interventions. One type of intervention is reoccurring and non-financial incentives, these differ from financial purchase incentives which are a one-time financial incentive associated with the purchase of a PEV. Reoccurring and non-financial incentives include special lane access for PEVs (e.g. HOV/carpool lanes, bus lanes), parking incentives, charging infrastructure development, road toll fee waivers, and licensing incentives. They also include disincentives such as gasoline tax or annual vehicle taxes. The impact of these incentives differs between regions partially due to differences in traffic conditions, travel patterns, consumer preferences, and other local variations. Due to these differences, it is challenging to rank the importance of these incentives, however existing research shows that they all can have a positive impact on PEV adoption. Policymakers wishing to promote the introduction of PEVs will need to consider local travel patterns, the regulatory environment, and consumer preferences to determine the most viable policy interventions for their region.

Cover page of Electrifying Ride-Sharing: Transitioning to a Cleaner Future

Electrifying Ride-Sharing: Transitioning to a Cleaner Future


Incentives for plug-in electric vehicles (PEVs) are typically designed to encourage broad consumer adoption of the new technology. However, maximizing the emissions benefits from electrifying the transportation sector also requires incentives targeted at stakeholders with high travel intensity, i.e., those with particularly high passenger occupancy and/or vehicle-miles traveled (VMT). This policy brief focuses on one such class of stakeholders: transportation network companies (TNCs) such as Uber and Lyft. It examines empirical data of electric vehicle use in TNCs and discusses research findings on the potential impacts of electrifying TNCs. It also raises important considerations for the development of future policy.

View the NCST Project Webpage

Cover page of Panel Study of Emerging Transportation Technologies and Trends in California: Phase 2 Data Collection

Panel Study of Emerging Transportation Technologies and Trends in California: Phase 2 Data Collection


Individual travel options are quickly shifting due to changes in sociodemographics, individual lifestyles, the increased availability of modern communication devices (smartphones, in particular) and the adoption of emerging transportation technologies and shared-mobility services. These changes are transforming travel-related decision-making in the population at large, and especially among specific groups such as young adults (e.g., “millennials”) and the residents of urban areas.

This panel study improves the understanding of the impacts of emerging technologies and transportation trends through the application of a unique longitudinal approach. The authors build on the research efforts that led to the collection of the 2015 California Millennials Dataset and complement them with a second wave of data collection carried out during 2018, generating a longitudinal study of emerging transportation trends with a rotating panel structure. The use of longitudinal data allows researchers to better assess the impacts of lifecycle, periods and generational effects on travel-related choices, and analyze components of travel behavior such as the use of shared mobility services among various segments of the population and its impact on vehicle ownership over time. Further, it helps researchers evaluate causal relationships between variables, thus supporting the development of better-informed policies to promote transportation sustainability.

View the NCST Project Webpage

Cover page of Framework for Urban Metabolism and Life Cycle Assessment of Hardscape

Framework for Urban Metabolism and Life Cycle Assessment of Hardscape


Urban hardscapes can be defined as human-altered surfaces in contact with the earth in urban areas other than alterations for horticulture. Hardscape covers large portions of the urban surface area, and has potentially large influence on air emissions, truck traffic and its associated problems, and the potential for flooding. Modeling the inflows of hardscape materials and the outflows of demolished hardscape and other rock-based products from buildings and other civil infrastructure is expected to provide a means to find solutions for reducing these flows and their impacts. Modeling of urban hydrology with respect to the effects of hardscape on surface and groundwater flows from precipitation is expected to provide a means to find solutions that will reduce the risk of flooding and improve groundwater recharge.

The goal of this white paper is to advocate that researchers and policy-makers use the analytical approach of combining urban (UM), material flow analysis (MFA) and elements of life cycle assessment (LCA) to measure and improve the efficiency of urban hardscape in large urbanized areas with respect to environmental impacts affecting global warming, safety and quality of life through use of alternative hardscape structure and materials and more permeable hardscape. The white paper provides details on the proposed UM-LCA framework. Additionally, several data sources and modeling tools were identified that can be used in the UM-LCA framework to quantify material and energy flows and environmental impacts including water flows. An effort was also made to identify data for a few of the cities in California in order to demonstrate parts of the data collection and presentation process. The framework developed is not limited to a single U.S. state, rather it can be used in any geographic region of the U.S.

View the NCST Project Webpage

Cover page of Framework for Life Cycle Assessment of Complete Streets Projects

Framework for Life Cycle Assessment of Complete Streets Projects


A multitude of goals have been stated for complete streets including non-motorized travel safety, reduced costs and environmental burdens, and creation of more livable communities, or in other words, the creation of livable, sustainable and economically vibrant communities. A number of performance measures have been proposed to address these goals. Environmental life cycle assessment (LCA) quantifies the energy, resource use, and emissions to air, water and land for a product or a system using a systems approach. One gap that has been identified in current LCA impact indicators is lack of socio-economic indicators to complement the existing environmental indicators. To address the gaps in performance metrics, this project developed a framework for LCA of complete streets projects, including the development of socio-economic impact indicators that also consider equity. The environmental impacts of complete streets were evaluated using LCA information for a range of complete street typologies. A parametric sensitivity analysis approach was performed to evaluate the impacts of different levels of mode choice and trip change. Another critical question addressed was what are different social goals (economic, health, safety, etc.) that should be considered and how to consider equity in performance metrics for social goals. This project lays the foundation for the creation of guidelines for social and environmental LCAs for complete streets.

View the NCST Project Webpage

Cover page of Guidance for Selection of Unbound Pavement Layer Seasonal Stiffnesses

Guidance for Selection of Unbound Pavement Layer Seasonal Stiffnesses


One of the benefits of using mechanistic-empirical (ME) design methods for pavements is the ability to calculate pavement response to various loading and climate conditions, and then in turn to model the entire damage process that is expected to occur over the pavement lifetime. One property that is currently not accounted for within California’s ME design software (CalME) is the change in stiffness of unbound materials that may occur due to seasonal moisture patterns. The engineering properties of unbound material may change due to a variety of factors, such as fluctuations in water content, changes in suction during wetting or drying periods, changes in overburden stress, and they are also dependent on geologic setting. Before moving to develop and implement more complex relationships to model assumed changes in the properties of unbound layers due to seasonal moisture changes, the University of California Pavement Research Center (UCPRC) evaluated the extent of variation that is observed in the field. The goals of this research are to evaluate whether or not these speculated seasonal changes in unbound material properties warrant further design optimization, and if so, how research to characterize such optimization should proceed in the future.

In this research, an experiment was performed to evaluate if noticeable changes in subgrade stiffness can be identified and explained using available, pertinent, and easy-to-use pavement monitoring equipment. Testing was performed twice on sections across California, once in the wet season and once in the dry season, to get a broad picture of the types of materials present and their corresponding properties during wet and dry seasons. Monitoring of a test section at UC Davis was also performed more frequently to observe changes in stiffness occurring after rainfall events and during wetting and drying cycles.

The literature and various laboratory experiments investigating the influence of moisture and suction on the resilient response of unbound materials strongly suggest that a large degree of variability in stiffness should be encountered in different moisture conditions. However, the results of the study revealed that a majority of the unbound material tested experienced minor, if any, changes at all in stiffness between the two rounds of testing seasonal testing. The most susceptible materials to stiffness variation were not necessarily the compacted subgrade material, but were the stabilized and unstabilized granular materials directly underlying the asphalt surface. While changes in moisture content and penetration resistance were observed between the two rounds of testing, they did not necessarily correspond to significant fluctuation in the field-tested stiffness of the unbound materials; rather, other factors such as spatial variability, drainage conditions, soil type, and influences from overlying layers tended to have a much larger influence on the resilient response of these materials than did seasonal moisture change.

It is therefore recommended that CalME’s current assumption of constant stiffness for unbound layers continue to be used, except in cases where the designer identifies issues with drainage, irrigation, or other likely causes of seasonal variation of stiffness. Performing FWD testing for backcalculation of unbound layer stiffnesses after the rainy season, or at other times of highest moisture contents where rainfall is not the main source of moisture, will impart some conservatism into designs.

UC Pavement Research Center Research Report UCPRC-RR-2017-11

Cover page of Status Review of California’s Low Carbon Fuel Standard, 2011–2018 Q1<em>
  <em>September 2018 Issue</em>

Status Review of California’s Low Carbon Fuel Standard, 2011–2018 Q1 September 2018 Issue


From 2011–2017, the share of alternative fuels in California’s transportation energy grew from 6.1 percent to 8.5 percent. Of alternative fuel energy, the portion coming from non-liquid fuels increased from 7.6 percent to 13.5 percent over the period.

Through 2018 Q1, total emissions reduction requirements under the regulation were 28.9 million tons (MMT) CO2e. Actual reported emissions reductions were 38.3 MMT CO2e, representing overcompliance of 9.3 MMT CO2e, creating a system-wide credit “bank” that can be used to meet future targets.

In 2017 and 2018 Q1, program deficits exceeded credits for the first time, by 0.1 MMT CO2e and 0.4 MMT CO2e, respectively, drawing down the credit “bank.”

Increases in alternative fuel use and declines in carbon intensity (CI) rating came primarily from the diesel pool. Biomass-based diesel—biodiesel and renewable diesel—accounted for 0.4 percent of liquid diesel fuel by volume in 2011 and 15.6 percent in 2018 Q1. Natural gas in transportation grew 111 percent from 2011–2017 to 178.1 gasoline gallon equivalent (gge). Of this natural gas, biogas use was close to nil in 2011 but approximately two-thirds in 2017.

Among gasoline substitutes, electricity use grew from less than 0.5 percent of alternative energy in 2011 to 4.5 percent in 2018 Q1. Use of ethanol, the largest renewable fuel by volume, remained close to a “blendwall” of 10 percent blended with gasoline.

Prices of LCFS compliance credits (each representing 1 MMT CO2e) fluctuated. Average per- credit price increased from $20 to $80 in 2013, ranged between $20 and $30 in 2014 and 2015 under a frozen standard of 1%, rose above $100 in 2016 when the freeze was lifted, and exceeded $160 in summer 2018 as the California Air Resources Board (CARB) was in the process of adopting more stringent targets for 2030.

LCFS amendments to be voted on at the September 26-27, 2018, CARB board meeting to take effect in 2019, include: a 2030 target of 20 percent CI reduction below 2010 levels; independent verification and monitoring of fuel pathway CI rating inputs; allowing alternative aviation fuel to generate program credits; a protocol for carbon capture and sequestration credits; credits for low- or zero-carbon intensity electricity use; requiring use of a portion of residential electricity credits to fund a statewide point-of-sale incentive program to electric vehicle (EV) buyers if such a program is approved by the California Public Utilities Commission; and introducing capacity credits for EV fast chargers and hydrogen fuel stations. The “capacity credit” provision would permit credit generation untied to current emissions reductions and favor particular fuels (those used in zero emission vehicles, which have no tailpipe emissions) for the first time.

LCFS-like programs are in development in Canada (a Clean Fuel Standard to  cover transportation, industry, and building sectors) and Brazil (the RenovaBio program focused on renewable liquid fuels and biogas). Neither plans to account for indirect land use change emissions in carbon intensity lifecycle analysis at program outset. Implementation of the Oregon and British Columbia LCFS programs is proceeding.

Click here to see all the California LCFS status reviews 

Cover page of A First Look at Vehicle Miles Travelled in Partially-Automated Vehicles

A First Look at Vehicle Miles Travelled in Partially-Automated Vehicles


This paper contributes to research investigating the impact of automated and partially automated vehicles on travel behavior. This contribution comes from taking a first look at the impact of partially/semi-automated (SAE Level 2) vehicles on travel behavior and potential correlations with vehicle miles travelled (VMT). The results of this study are taken from a questionnaire survey of 3,001 plug-in electric (PEV) owners in the USA, of which 347 own a partially-automated vehicle (e.g Tesla Model S with Autopilot). This study looks at the VMT of different vehicle types in the survey including plug-in hybrids (PHEVs), battery electric vehicles (BEVs), and semi-automated BEVs. This comparison reveals that semi-automated BEVs have significantly higher VMT compared to other vehicle types. Least squares regression is used to understand VMT in semi-automated BEVs further. This reveals a significant relationship between commute distance, age, household income, house type, and the frequency of autopilot use, and annual VMT. It is possible that the results are showing a self-selection causality as owners of these vehicles already drove more prior to them selecting a semi-automated BEV. Nevertheless, this model indicates that as the frequency of autopilot use increases, so does annual VMT. Due to the potential for two ways causality this study cannot determine whether there is a causal relationship between the use of semi-automated vehicle technology and additional VMT. It is hoped that this first look at the impact of partially-automated BEVs will encourage more research and debate in this area with the aim of improving policy responses to partially and fully automated vehicles.