Browsing by Subject "Transport"

Now showing 1 - 20 of 56

Accessibility and the evaluation of investments on the Beijing subway
(Journal of Transport and Land Use, 2017) Jiang, Haibing; Levinson, David
This study measures the job and population accessibility via transit for Beijing using the cumulative opportunity metric. It is shown that transit accessibility varies widely across Beijing, but is highly focused on subway stations. Early lines added far more accessibility than more recently planned lines.
An Agent-Based Model of Origin Destination Estimation (ABODE)
(Journal of Transport and Land Use, 2013) Tilahun, Nebiyou; Levinson, David
This paper proposes and tests an agent-based model of worker and job matching. The model takes residential locations of workers and the locations of employers as exogenous and deals specifically with the interactions between firms and workers in creating a job-worker match and the commute outcomes. It is meant to illustrate that by explicitly modeling the search and hiring process, origins and destinations (ODs) can be linked at a disaggregate level. The model is tested on a toy-city as well as using data from the Twin Cities area. The toy-city model illustrates that the model predicts reasonable commute outcomes, with agents selecting the closest work place when wage and skill differentiation is absent in the labor market. The introduction of wage dispersion and skill differentiation in the model increases the the average home to work distances considerably. Using data from Twin Cities area of Minneapolis-St. Paul, aggregate commute and wage outcomes from the model are shown to capture the trends in the observed data. Overall, the results suggest that the behavior rules as implemented lead to reasonable patterns. Future directions are also discussed.
An application of the node-place model to explore the spatial development dynamics of station areas in Tokyo
(Journal of Transport and Land Use, 2011) Chorus, Paul; Bertolini, Luca
The high level of integration between railway and real estate development in Tokyo makes the city an interesting example for other metropolitan areas looking for ways to promote transit-oriented development. To successfully promote such a development pattern, an understanding of development dynamics in station areas is crucial. In this paper, a node place model is used to determine which transport and land use factors are responsible for structuring station area redevelopments in Tokyo, and to what extent. The interaction between specific transport and land use features—most importantly, proximity by train to the central business district and the number of train connections versus workforce concentration—is a powerful force structuring developments in Tokyo. However, other factors—most notably government policies—should also be taken into account.
Assessing the reality—Transport and land use planning to achieve sustainability
(Journal of Transport and Land Use, 2012) Banister, David
This paper takes a historical perspective on how cities have become less sustainable in terms of transport, but it will argue that many positive changes have taken place even before the current concerns over CO2 and oil. There seem to be many more opportunities for further change through the encouragement of high-quality city-based lifestyles that do not require high levels of carbon-based mobility. But it is in the newly emerging “megacities” that the main problems occur, as there is a discontinuity between the slow growing, stable, and well-structured cities of the west and the rapidly growing, unstable, and unstructured cities of the east.
Automatic street widening: Evidence from a highway dedication law
(Journal of Transport and Land Use, 2017) Manville, Michael
Cities often require developers to widen streets or make other transportation improvements to account for the traffic impacts of new building. This article examines one parcel-level traffic mitigation law in depth—the highway dedication ordinance of the city of Los Angeles. I first show that the law emerged from a combination of happenstance and political and fiscal constraints, not from persuasive evidence it would be effective. I then show that the traffic predictions underlying the law are often inaccurate, and that, in fact, the standards the law is based on are in some ways unverifiable. Thus the law likely does little to reduce congestion and probably impedes housing development. Finally, I argue that the law persists precisely because its desired outcome is hard to verify: Without measurable goals, planners fall back on a measurable process. Parcel-level traffic mitigation becomes an exercise not in reducing traffic but in ensuring that developers carry out mitigations, regardless of whether those mitigations are effective.
Book Review: Planning for Place and Plexus
(Journal of Transport and Land Use, 2008) Handy, Susan L.
This article reviews the book Planning for Place and Plexus: Metropolitan Land Use and Transport by David M. Levinson and Kevin J. Krizek (Routledge, New York; 2008).
Bridging scales in modeling and simulation of thermal transport processes
(2014-08) Wheeler, Vincent Michael
The vastly disparate length and time scales existing in new devices and materials born out of nanotechnology have made thermal modeling and simulation more important and more difficult. The experimental thermal characterization of such systems, e.g. modern computer processors, can be prohibitively difficult or expensive making numerical simulation the only route to effective technology design. However, obtaining solutions that account for small scales, but are still computationally feasible, requires innovative modeling approaches. The research contained herein represents three independent contributions to the understanding of the modeling of thermal transport processes in systems with nano-sized features. At their common core, all contributions in this thesis are rooted in transport theory--the solution or approximation of the Boltzmann equation (BE)--to statistically describe a system made up of a great many energy-carrying particles. The work roughly divides into the three modes of heat transfer--convection, conduction, and radiation. First, a framework for the discretization of the BE (in its many forms) based on lattices is presented. The widely-used lattice Boltzmann method for the simulation of fluid flow is shown to be a sub-case. The framework gives a new rigorous foundation to the use of lattice methods which have emerged in recent years with applications ranging from Brownian motion to astrophysical radiation. Second, we give a thorough presentation of recently proposed models of heat conduction derived from the phonon BE which provides rigor and insight into the different approaches. Most notably, the "new heat equation" is derived directly from the phonon BE for the first time along with a novel boundary condition. The result is shown to give excellent agreement with the more detailed description provided by the equation of phonon radiative transport. Last, we provide the radiative characterization of a nano-porous material using Maxwell's equations in order to recover coefficients to the linear BE governing thermal radiative transfer.
Can measuring the benefits of accessible transport enable a seamless journey?
(Journal of Transport and Land Use, 2009) Maynard, Alice
For disabled and older people, journeys need to be seamless – with no failures in access from origin to destination. Because the public transport environment, including walking and cycling modes, is not accessible, the use of private cars remains essential to social inclusion. Consequently, social goals re- lating to private car use in relation to health, environment and land-use will be harder to achieve. Greater attention needs to be paid to the detail of the “journey chain” with access consistently provided through- out, making for seamless journeys. This attention needs to be paid in all aspects of transport planning as well as delivery, including in the appraisal process. In transport projects appraisal, the costs of providing access are monetized, but not the benefits. The author undertook an experiment to value the benefits of step-free access for everyone and found significant economic benefit that enhanced the benefit:cost ratio. Until the benefits of accessible transport are properly considered for everybody across the whole planning process including appraisal, providing access will continue to be an uphill struggle and access throughout the journey chain will remain hit and miss.
China motorization trends: New directions for crowded cities
(Journal of Transport and Land Use, 2010) Ng, Wei-Shiuen; Schipper, Lee; Chen, Yang
This paper examines two major emerging constraints on transport in fast-growing Chinese urban cities: oil supply and urban infrastructure. The research considers automobile technology, alternative fuels, and mobility choices, as well as policy measures that could be adopted to reduce the use of oil for transport and greenhouse gas emissions. Three transport energy scenarios, “Road Ahead,” “Oil Saved,” and “Integrated Transport,” illustrate potential motorization trends given different policy, vehicle technology, alternative fuels, and driving-behavior assumptions. In the Integrated Transport scenario, where congestion and space constraints favor small and vehicles moving at slower speeds, gasoline and electric cars are the highest in use. Oil consumption in the Integrated Transport scenario is only 12 percent of its value in Road Ahead by 2020, while carbon emission is 79 percent lower. Policies such as vehicle technology and fuel requirements, while important, are not as crucial as integrated land use development, taxation of vehicle use, road pricing, and the prioritization of public and non-motorized transport that could trigger a world of fewer, smaller and more efficient cars. According to experiences around the world, fuel and carbon dioxide concerns alone are not strong enough to promote a change in the path of individual motorization.
Cities as Organisms: Allometric Scaling of Urban Road Networks
(Journal of Transport and Land Use, 2008) Samaniego, Horacio; Moses, Melanie E.
Just as the cardiovascular network distributes energy and materials to cells in an organism, urban road networks distribute energy, materials and people to locations in cities. Understanding the topology of urban networks that connect people and places leads to insights into how cities are organized. This paper proposes a statistical approach to determine features of urban road networks that affect accessibility. Statistics of road networks and traffic patterns across 425 U.S. cities show that urban road networks are much less centralized than biological vascular networks. As a result, per capita road capacity is independent of the spatial extent of cities. In contrast, driving distances depend on the size of the city, although not as much as is predicted by a completely centralized model. This intermediate pattern between centralized and decentralized extremes may reflect a mixture of different travel behaviors. The approach presented here offers a novel macroscopic perspective on the differences between small and large cities and on how road infrastructure and traffic might change as cities grow.
The coevolution of transport and land use: An introduction to the Special Issue and an outline of a research agenda
(Journal of Transport and Land Use, 2011) Levinson, David
This article introduces vol. 4, no. 2 issue of Journal of Transport and Land Use. This issue focuses on coevolution: how transport drives changes in land use, and vice versa. The issue contains four research articles, examining different geographies, eras, and technologies. These papers present new findings, but as good science should, raise new questions, and help us set a research agenda to better understand the coevolution of transport and land use.
Compulsory convenience? How large arterials and land use affect midblock crossing in Fushun, China
(Journal of Transport and Land Use, 2010) Tao, Wendy; Mehndiratta, Shomik; Deakin, Elizabeth
This study focuses on how street design and land uses influence pedestrian behavior in a medium-sized Chinese city, Fushun. In cities throughout China, the change from workplace-managed and assigned housing to market housing has had profound effects on pedestrians. Coupled with motorization, pedestrian trips are increasingly external, pushed out of the protected space of the gated block and onto massive arterials that now carry automobiles, trucks, and buses in growing numbers. Long blocks, unenforced zebra crossings, and inadequate green time at traffic signals do not equitably accommodate those on foot; thus, pedestrians violate the system by crossing midblock. In Fushun, the long block lengths and large arterials, lack of enforcement, and unfavorable pedestrian policies creates an environment which incentivizes midblock crossing behavior.
Constraints in household relocation: Modeling land-use/transport interactions that respect time and monetary budgets
(Journal of Transport and Land Use, 2017) Moeckel, Rolf
Traditionally, integrated land-use/transportation models intend to represent all opportunities of travel and household location, maximize utilities and find an equilibrium in which no person or household could improve their satisfaction any further. Energy scarcity, higher transportation costs, and an increasing share of low-income households, on the other hand, demand special attention to represent constraints that households face, rather than opportunities for utility maximization. The integrated land-use model SILO explicitly represents various constraints, including the price of a dwelling, the travel time to work, and the monetary transportation budget. SILO ensures that no household makes choices that violate these constraints. Implementing such constraints helps SILO to generate more realistic results under scenarios that put current conditions under a stress test, such as a serious increase in transportation costs or severely increased congestion.
Counterpoint: Accessibility and Sprawl
(Journal of Transport and Land Use, 2008) Crane, Randall
This essay provides a counterpoint to Robert Bruegmann's perspective on accessibility and sprawl in this journal volume. Bruegmann's recent work on the history of urban form situates contemporary discussions of sprawl in a well-researched historical context; however, this essay takes a different perspective on several key points in Bruegmann's analysis, particularly in relation to cost-benefit analysis for transportation-disadvantaged populations.
Development of a Novel Perfluoroalkyl Substance Sequestration Scheme Using Alginate Macrobeads and Common Water Treatment Polymers
(2016-09) McCarty, Michael
Groundwater contamination by perfluoroalkyl substances (PFASs) is a topic of growing concern. Preventing transport via sorption-enhancement to aquifer material is a potential solution to this problem. Two models of osmotic surgical implant pumps were used to demonstrate that the cationic polymers poly(diallyldimethylammonium chloride) (polyDADMAC) and epichlorhydrin dimethylamine (EpiDMA) could be reliably and passively delivered to promote sequestration of PFASs. It was found that pumps behaved as expected with one failure near the end of pump lifespan. Next, alginate was investigated as an encapsulation media for EpiDMA and polyDADMAC to develop a bead-like vector for delivery of these compounds in a time release manner. Bead experiments examined alginate synthesis concentration, membrane thickness, and alginate composition. Synthesis concentration had no effect on mass transfer. Beads synthesized with greater membrane thickness may be characterized by slower polymer release, and alternative alginate composition results were promising. A loss on ignition scheme was also used to evaluate the effectiveness of polyDADMAC as a sorption enhancer for the PFASs perfluoroctanesulfonate (PFOS), perfluorooctanoate (PFOA), and perfluorononanoate (PFNA) and to elucidate potential removal mechanisms. In all cases, sorption of compounds increased beyond what can be explained by hydrophobic partitioning indicating the presence of coulombic attraction between the contaminant and cationic polymer. Finally, sand and soil column studies were used to assess the behavior of beads containing polyDADMAC and the transport potential of PFOS in a dynamic environment with sorption enhancer present. PolyDADMAC exhibited little retention on Ottawa sand but significant retention on soil excavated from Tinker Air Force Base. Polymer retention, however, was less than indicated by batch studies and is likely inhibited by steric and kinetic factors.
Development of computational tools for modeling the biotransport of small organic molecules into the active site of broad-substrate specificity enzymes
(2019-07) Escalante, Diego
In this dissertation research, two new computational tools were developed to model the biotransport of small organic molecules into the active site of broad-substrate specificity (BSS) enzymes. The biological organism selected to develop, test and validate these tools were Rieske non-heme iron dioxygenases. Members of this family of enzymes are known to have biocatalytic activity on more than three hundred different substrates. The large diversity of substrates that can be acted upon makes these enzymes very attractive in biotechnological processes such as bioremediation. In addition, the highly specific chirality of the products obtained makes these enzymes attractive for the potential synthesis of pharmaceutical precursors. Currently, the most common way to identify new substrates requires formulating an educated guess followed by the arduous task of testing each possible compound individually. This slows down the pace at which new industrial processes can be formulated or current ones further developed. The tools presented in this research provide fundamental and practical scientific contributions. For the basic science studies of my dissertation, an all-atom and, a coarse-grained (CG) model of Rieske non-heme iron dioxygenases were used to investigate the factors that affect the biotransport of small organic molecules into their active sites. From the all-atom model I discovered a gating mechanism that allows aromatic substrates into the active site and blocks other compounds. The key to these gates are T-stacked pi-pi interactions between hydrophobic amino acids and the aromatic substrates. On the other hand, from the CG model I discovered that the shape of tunnel modulates the hydrophobicity level of the surface. As the tunnels become more concave, the hydrophobicity increases causing the formation of a water exclusion zone which increases the diffusivity of aromatic substrates. The CG models also revealed that convex tunnels prevent the adhesion of hydrophobic substrates to the tunnel walls; providing a possible explanation for the evolution of bottlenecks at the entrance of Rieske active sites. For the practical contributions of my dissertation, I developed two new computational tools for the prediction of Rieske substrates. The first tool is an all-atom algorithm that models the stochastic roto-translational movement of small organic molecules along the Rieske enzyme tunnels. This algorithm has a 92% prediction accuracy of Rieske substrates. In addition, it is capable of elucidating the location of high-energy barriers along the tunnel, allowing the formulation of possible protein engineering sites. The second tool is a CG non dimensional model of the Rieske enzyme tunnels. This algorithm has a 90% prediction accuracy of Rieske substrates. The processing time of 1ms/substrate combined with its high accuracy allows for the high-throughput screening of possible Rieske substrates.
Does first last? The existence and extent of first mover advantages on spatial networks
(Journal of Transport and Land Use, 2011) Levinson, David; Xie, Feng
This paper examines the nature of first-mover advantages in the deployment of spatially differentiated surface transport networks. A number of factors explaining the existence of first-mover advantages have been identified in the literature; however, the questions of whether these factors exist in spatial networks, and of how they play out with true capital immobility have remained unanswered. By examining empirical examples of commuter rail and the Underground in London, first-mover advantage is observed and its sources explored. A model of network diffusion is then constructed to replicate the growth of surface transport networks, making it possible to analyze first-mover advantage in a controlled environment. Simulation experiments are conducted, and Spearman rank correlation tests reveal that first-mover advantages can exist in a surface transport network and can become increasingly prominent as the network expands. In addition, the analysis discloses that the extent of first-mover advantages may relate to the initial land use distribution and network redundancy. The sensitivity of simulation results to model parameters are also examined.
Does public transit use enhance the economic efficiency of urban areas?
(Journal of Transport and Land Use, 2012) Drennan, Matthew; Brecher, Charles
This paper explores the relationship between the scale of public transit services in large urban areas of the United States and the efficiency of those economies, with efficiency measured by commercial office rents. Panel regressions are estimated in which real office rent is the left-hand variable. The key right-hand variable is per-capita transit use. Other right-hand variables include demand for office space, office vacancy rate, average real wage, and unemployment rate. Two-stage least-squares equations are estimated to deal with possible simultaneity between office rents and transit use. Results indicate a positive relationship between public transit use and office rents. The relationship is positive and significant in urban areas with higher concentrations of office space in the central business district, and nonexistent in urban areas with lower concentrations. The estimated dollar impact of transit use on office rents is small.
The egress of fluid from the brain via arachnoid transport: foundational work for the tissue engineering of the arachnoid granulation
(2011-12) Lam, Cornelius Hoktsim
The arachnoid tissue is a critical component for the removal of cerebrospinal fluid (CSF) and other substances. Failure results in hydrocephalus, increased intracranial pressure, and buildup of toxic materials in the brain. The purpose of this thesis is to establish a foundation for a biomimetic arachnoid construct. First, we characterized arachnoid cell transport in culture and on three-dimensional collagen scaffolds. Arachnoid cells were harvested from rat brainstems and cultured onto bilayered bovine collagen scaffolds. Cells exhibited arachnoid cell phenotype (positive for vimentin, desmoplakin, and cytokeratin), readily penetrated the collagen scaffold, and doubled approximately every 2–3 days. The transepithelial electrical resistance for a monolayer of cells was 160 Ω∙cm2, and permeability of indigo carmine was 6.7+1.1X10- 6 cm/s. Hydraulic conductivity of the collagen construct was 6.39 mL/min/mmHg/cm2. Because of practical limitations of primary culture which include slow growth, early senescence, and poor reproducibility, we created two immortalized rat arachnoid cell lines using retroviral gene transfer of SV40 large T antigen (SV40 LTAg) either with or without human telomerase (hTERT). They stably expressed either SV40 LTAg alone, or SV40 LTAg and hTERT, and demonstrated high proliferative rate, contact inhibition at confluence, and stable expression of protein markers characteristic of native arachnoid cells for more than 160 passages. We subsequently used them to determine arachnoidal barrier properties and paracellular transport. Permeabilities of urea, mannitol, and inulin were 2.9+1.1X10-6, 0.8+.18x10-6, and 1.0+.29x10-6 cm/s respectively. Size differential permeability testing with dextran clarified the arachnoidal blood-CSF-barrier limit and established a rate of intracellular transport to be two orders of magnitude slower than paracellular transport in a polyester membrane diffusion chamber. The theoretical pore size for paracellular space was 11Å and the occupancy to length ratios were 0.8 and 0.72 cm-1 for urea and mannitol respectively. The monolayer permeability was not significantly different from an apical to basal direction or vice versa. Gap junction may have a role in barrier formation. Although up-regulation of claudin by dexamethasone did not significantly alter paracellular transport, increasing intracellular cAMP decreased mannitol permeability. Calcium modulated paracellular transport, but only selectively with the ion chelator, EDTA, and with disruption of intracellular stores. Without the neurovascular unit of the blood-brain-barrier, the blood-CSF-barrier at the arachnoid tissue is anatomically and physiologically different from the vascular based blood-brain-barrier. These studies provide a three dimensional architecture, a stable cellular substrate, and baseline blood-CSF-barrier properties for the establishment of a viable bioartificial arachnoid shunt.
Electron transport and recombination in nanowire dye-sensitized solar cells.
(2010-02) Enache-Pommer, Emil
The dye-sensitized solar cell (DSSC) is a promising low cost photovoltaic device. A typical DSSC consists of a porous film made out of TiO2 nanoparticles, a monolayer of dye adsorbed on the TiO2 surface and a liquid electrolyte. The electrolyte fills the pores of the nanoparticle film forming a semiconductor-dye-electrolyte interface with large surface area. During illumination of the cell, the dye molecules inject electrons into the TiO2 nanoparticles. The injected electrons diffuse through the nanoparticle network by hopping from particle to particle until they are collected at a transparent conductive oxide (TCO) anode. Meanwhile, the charged dye molecules are reduced through an electrochemical reaction with a reductant in the electrolyte. The oxidized ionic species diffuse to the counter electrode and are reduced by electrons that have been collected at the anode and have traveled through the load to complete the circuit. Currently, dye-sensitized solar cells have reached efficiencies above 11 %, but further improvement is limited by electrons recombining with the electrolyte during their transport through the semiconductor nanoparticle network. Nanowire DSSCs have been recently introduced and have the potential to overcome the limitations of nanoparticle DSSCs, since the electron percolation through the nanoparticle network is replaced by a direct electron pathway from the point of injection to the TCO. Understanding the electron transport and recombination mechanisms in nanowire DSSCs is one of the key steps to improving DSSC efficiency. Towards this end polycrystalline TiO2, single-crystalline TiO2 and single crystalline ZnO nanowire DSSCs were fabricated and analyzed using current-voltage characteristics, optical measurements, and transient perturbation techniques such as intensity modulated photocurrent spectroscopy, photocurrent decay and open-circuit photovoltage decay. For single-crystal ZnO nanowire DSSCs, the measured electron transport time constants are independent of light intensity but change with nanowire length, seeding method and annealing time. Even if the measured transients are limited by the RC time constant of the solar cell, using the measured time constants as an upper limit for the actual electron transport time leads to the conclusion that the electron transport rate in ZnO nanowires is at least two orders of magnitude faster than the recombination rate. This indicates that the charge collection efficiency in ZnO nanowire DSSCs is nearly 100 %. These results show that films can be made out of 100 μm long ZnO nanowires while maintaining efficient charge collection. For DSSCs based on polycrystalline anatase TiO2 nanowires, the electron transport times show a power-law dependence on illumination intensity similar to that reported for TiO2 nanoparticle DSSCs. The magnitude of the electron transport times is also comparable to that of nanoparticle DSSCs, indicating that electron trapping and detrapping determine transport times for polycrystalline TiO2 nanowire DSSCs. Surprisingly, even for single-crystal rutile TiO2 nanowire DSSCs, the electron transport rate is on the order of the electron transport rate in nanoparticle-based DSSCs and not as fast as would be expected. Electron transport is slow and light intensity dependent indicating that trapping and detrapping, most likely in surface traps, still play an important role in electron transport even in single-crystal rutile TiO2 nanowires.

University Digital Conservancy

Browse by Subject

Browsing by Subject "Transport"