conference paper
Phd Dissertation
This dissertation focuses on the development of optimization methods and approximation algorithms for combinatorial auctions, particularly with application to the contract procurement problem in freight transportation. Combinatorial auctions are auctions in which a set of heterogeneous items are sold simultaneously and in which bidders can bid for their preferred combinations of items. They involve many difficult optimization problems both for auction hosts and bidders and have received significant attention from computer scientists, operations researchers and economists recently. Large shippers (typically manufacturing companies or retailers) have begun to use this method to procure services from trucking companies and logistics services providers. This dissertation first analyzes the economic impact of combinatorial auction-based procurement methods both on shippers and carriers using a simulation study and reveals that both parties can benefit from this economically efficient price discovery mechanism. While the majority of prior research has been from an auctioneer’s perspective, we demonstrate that bidders have even more complicated optimization problems in combinatoiral auctions. The bid construction problem, that is, how bidders should identify and construct beneficial bids, is very hard and remains an open question. This dissertation investigates this problem and proposes an optimization based approximation method that involves solving an NP-hard problem only once, yielding significant improvements in computational efficiency. Further, the current state of trucking and third party logistics industries are examined. The trucking industry is very competitive and small carriers are operating under thin margins. This dissertation addresses these issues by proposing an auction based collaborative carrier network in which participating carriers can identify inefficient lanes from daily operations quickly and exchange them with partners under an auction protocol. This system is proved to be Pareto efficient. Further, decision problems are discussed regarding how carriers should identify inefficient operations and how to make and select bids. This represents an effort to use advanced auction mechanisms to enhance the carriers’ operational efficiencies.
published journal article
published journal article
working paper
This working paper reports initial results of a set of analyses investigating differences between non-truck involved and truck-involved crashes. Data was selected from the California Department of Transportation TASAS accident reporting system. Results indicate that there are differences in primary collision factors (as assigned by the California Highway Patrol through issuance of traffic citations) between truck-involved and non-truck-involved crashes in the vicinity of freeway interchanges. “Speeding’ in truck-involved crashes is cited at approximately one-half its rate for non-truck-involved crashes. Additionally, there is no statistical difference in the appearance of the “Uninvolved Motorist” TASAS party designation between truck and non-truck-involved crashes.
published journal article
One approach to making transportation more sustainable is to transition away from a car-oriented society. Unfortunately, our understanding of the factors that prompt households to voluntarily forgo their motor vehicles is limited. The 2008 Mobility in Germany (MiD) and the 2012 California Household Travel Survey (CHTS) provide an opportunity to start filling this gap by teasing out what built environment and socio-economic variables impact the likelihood that a household is carless (voluntarily or not) in Germany and in California, two car-dependent societies with different carless rates. Results from our generalized structural equation models show that in both Germany and California, households who reside in denser neighborhoods, closer to transit stations, and who have a lower income or fewer children, are more likely to be voluntarily carless. However, households with more education are more likely to be voluntarily carless in Germany, whereas the reverse is true in California. Moreover, employment density and public transit have a higher impact on voluntary carlessness in Germany than in California. Our results also show that different socio-economic groups have substantially different residential location preferences in Germany and in California. These differences may be explained by cultural preferences, historical differences in land use and transportation policies, and by the higher cost of owning a motor vehicle in Germany.
Preprint Journal Article
The environmental and resilience concerns and the potential mass adoption of electric vehicles (EVs) call for managing and operating power-mobility systems (PMS) as a whole. Agent-based and network flow-based modeling methods have proven useful for potential model-based PMS management and operation, but they tend to be heavy in computation, lack intuitive implications of overall PMS dynamics, and are inflexible to the change of the problem scope and resolution. These challenges call for a complementary modeling approach to facilitate model-based large-scale urban PMS decision-making. To this end, we propose a dynamic macroscopic modeling framework for high-level strategic decision-making. The framework contains three main components – power production and delivery, energy exchange interface between power grids and EVs, and EV-involved mobility systems. A case study shows that this approach facilitates tractable and coherent model development with low computational requirements and shows promising benefits of coordinated PMS.
published journal article |
Jul 2022
published journal article |
Jan 2025
published journal article |
Sep 2023
working paper
We extend a previous cost-effectiveness analysis of methanol versus other means of controlling emissions from urban transit buses, by developing a method to incorporate their effects on two end-product pollutants: ozone and nitrogen dioxide. Using published simulation results from an airshed grid model of ozone formation, we find that the measures we consider have varying effects on ozone at 23 sites in the Los Angeles air basin. The effects are offsetting, leading to a negligible net effect when aggregated across the basin’s population; this is true assuming either that damage is proportional to concentration times population exposed, or that damage is represented by nonlinear concentration-response functions for specific health conditions. In contrast, either low-aromatic diesel fuel or methanol would lower ambient concentrations of nitrogen dioxide enough, relative to the federal or California ambient standard, to significantly affect cost-effectiveness comparisons.
published journal article
The goal of this study is to assess and quantify the potential employment accessibility benefits of shared-use automated vehicle (AV) mobility service (SAMS) modes across a large diverse metropolitan region considering heterogeneity in the working population. To meet this goal, this study proposes employing a welfare-based (i.e. logsum-based) measure of accessibility, obtained via estimating a hierarchical work destination-commute mode choice model. The employment accessibility logsum measure incorporates the spatial distribution of worker residences and employment opportunities, the attributes of the available commute modes, and the characteristics of individual workers. The study further captures heterogeneity of workers using a latent class analysis (LCA) approach to account for different worker clusters valuing different types of employment opportunities differently, in which the socio-demographic characteristics of workers are the LCA model inputs. The accessibility analysis results in Southern California indicate: (i) the accessibility benefit differences across latent classes are modest but young workers and low-income workers do see higher benefits than high- and middle-income workers; (ii) there are substantial spatial differences in accessibility benefits with workers living in lower density areas benefiting more than workers living in high-density areas; (iii) nearly all the accessibility benefits come from the SAMS-only mode as opposed to the SAMS+Transit mode; and (iv) the SAMS cost per mile assumption significantly impacts the magnitude of the overall employment accessibility benefits.
research report |
Feb 2020
published journal article
Background The frequency and severity of wildfires have been sharply increasing due to climate change, which largely contributes to ambient particulate matter (PM) pollution. We conducted a systematic review focusing on the short-term relationships between PM attributable to wildfires (wildfire-specific PM) and diverse respiratory endpoints, with a comparison between the effects of wildfire-specific PM vs. all-source/non-wildfire PM. Methods A comprehensive online search for the literature published from 2000 to 2022 was conducted through PubMed, Web of Sciences, Scopus, and EMBASE. We applied search terms related to wildfire smoke and respiratory health outcomes. Results In total, 3196 articles were retrieved, and 35 articles were included in this review. Most studies focused on the associations of wildfire-specific PM with an aerodynamic diameter of ≤2.5 μm (PM2.5) with respiratory emergency department visits or hospitalizations, with a time-series or case-crossover study design. Studies were mostly conducted in the United States, Canada, and Australia. Positive associations of wildfire-specific PM with respiratory morbidity were observed in most studies. Studies that focused on respiratory mortality were limited. Females can be more vulnerable to the respiratory impacts of wildfire PM, while the evidence of vulnerable subpopulations among different age groups was inconclusive. Few studies compared the effects of wildfire-specific vs. all-source/non-wildfire PM, and some reported higher levels of toxicity of wildfire-specific PM, potentially due to its distinct chemical and physical compositions. Asthma and chronic obstructive pulmonary disease were the most studied diseases, and both were adversely affected by wildfire-specific PM. Conclusion To our knowledge, this is the first review that systematically summarized the associations of wildfire-specific PM exposure with adverse respiratory outcomes and compared associations of wildfire-specific vs. all-source/non-wildfire PM. Further investigations may add to the literature by examining the impacts on respiratory mortality and the effects of specific PM components from different types of wildfires.
