Doctor of Philosophy Theses
- Modeling Intercity Mode Choice and Airport Choice in the United States (2007)
Author: Dr. Senanu Ashiabor
The aim of this study was to develop a framework to model travel choice behavior in order to estimate intercity travel demand at nation-level in the United States. Nested and mixed logit models were developed to study national-level intercity transportation in the United States. A separate General Aviation airport choice model to estimates General Aviation person-trips and number of aircraft operations though more than 3000 airports was also developed. The combination of the General Aviation model and the logit models gives the capability to estimate a full spectrum of intercity travel demand in the United States.
The logit models were calibrated using a nationwide revealed preference survey (1995 American Travel Survey). Separate models were developed for business and non-business trip purposes. An airport choice model is integrated into the mode choice model to estimate both the market share between any origin-destination pair and other modes of transportation, and the market share split between airports associated with the origin-destination pairs. The explanatory variables used in the utility functions of the models are travel time, travel cost, and traveler’s household income. The logit models are used to estimate the market share of automobile and commercial air transportation between 3091 counties and 443 commercial service airports in the United States. The model was also used to estimate market share for on-demand air taxi services. Given an input county-to-county trip demand table, the models were used to estimate county-to-county travel demand by automobile and commercial airline between all counties and commercial service airports in the United States. The model has been integrated into a computer software framework called the Transportation Systems Analysis Model (TSAM) that estimates nationwide intercity travel demand in the United States.
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- Modeling Light Duty Vehicle Emissions Based on Instantaneous Speed and Acceleration Levels (2002)
Author: Dr. Kyoungho Ahn
This dissertation develops a framework for modeling vehicle emissions microscopically. In addition, the framework is utilized to develop the VT-Micro model using a number of data sources. Key input variables to the VT-Micro model include instantaneous vehicle speed and acceleration levels. Estimating accurate mobile source emissions is becoming more and more critical as a result of increasing environmental problems in large metropolitan urban areas. Current emission inventory models, such as MOBILE and EMPAC, are designed for developing large scale inventories, but are unable to estimate emissions from specific corridors and intersections. Alternatively, microscopic emission models are capable of assessing the impact of transportation scenarios and performing project-level analyses.
The VT-Micro model was developed using data collected at the Oak Ridge National Laboratory (ORNL) that included fuel consumption and emission rate measurements (CO, HC, and NOx) for five light-duty vehicles (LDVs) and three light-duty trucks (LDTs) as a function of the vehicle¡¯s instantaneous speed and acceleration levels. The hybrid regression models predict hot stabilized vehicle fuel consumption and emission rates for LDVs and LDTs. The model is found to be highly accurate compared to the ORNL data with coefficients of determination ranging from 0.92 to 0.99. The study compares fuel consumption and emission results from MOBILE5a, VT-Micro, and CMEM models. The dissertation presents that the proposed VT-Micro model appears to be good enough in terms of absolute light-duty hot stabilized normal vehicle tailpipe emissions. Specifically, the emission estimates were found to be within the 95 percent confidence limits of field data and within the same level of magnitude as the MOBILE5a model estimates. Furthermore, the proposed VT-Micro model was found to reflect differences in drive cycles in a fashion that was consistent with field observations. Specifically, the model accurately captures the increase in emissions for aggressive acceleration drive cycles in comparison with other drive cycles.
The dissertation also presents a framework for developing microscopic emission models. The framework develops emission models by aggregating data using vehicle and operational variables. Specifically, statistical techniques for aggregating vehicles into homogenous categories are utilized as part of the framework. In addition, the framework accounts for temporal lags between vehicle operational variables and vehicle emissions. Finally, the framework is utilized to develop the VT-Micro model version 2.0 utilizing second-by-second chassis dynamometer emission data for a total of 60 light duty vehicles and trucks.
Also, the dissertation introduces a procedure for estimating second-by-second high emitter emissions. This research initially investigates high emitter emission cut-points to verify clear definitions of high emitter vehicles (HEVs) and derives multiplicative factors for newly developed EPA driving cycles. Same model structure with the VT-Micro model is utilized to estimate instantaneous emissions for a total of 36 light duty vehicles and trucks.
Finally, the dissertation develops a microscopic framework for estimating instantaneous vehicle start emissions for LDVs and LDTs. The framework assumes a linear decay in instantaneous start emissions over a 200-second time horizon. The initial vehicle start emission rate is computed based on MOBILE6¡¯s soak time function assuming a 200-second decay time interval. The validity of the model was demonstrated using independent trips that involved cold start and hot start impacts with vehicle emissions estimated to within 10 percent of the field data.
The ultimate expansion of this model is its implementation within a microscopic traffic simulation environment in order to evaluate the environmental impacts of alternative ITS and non-ITS strategies. Also, the model can be applied to estimate vehicle emissions using instantaneous GPS speed measurements. Currently, the VT-Micro model has been implemented in the INTEGRATION software for the environmental assessment of operational-level transportation projects.
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- Development of Optimization and Simulation Models for the Analysis of Airfield Operations (2000)
Author: Dr. Hojong Baik
This research is concerned with the modeling and development of algorithmic approaches for solving airport operational problems that arise in Air Traffic Control (ATC) systems within the terminal area at hub airports. Specifically, the problems addressed include the Aircraft Sequencing Problem (ASP) for runway operations, the Network Assignment Problem (NAP) for taxiway operations, and a simulation model for the evaluation of current or proposed ATC system in detail.
For the ASP, we develop a mathematical model and apply the Reformulation-Linearization-Technique (RLT) of Sherali and Adams to construct an enhanced tightened version of the proposed model. Since ASP is NP-Hard and in fact, it is a variation of the well-known Traveling Salesman Problem with time-windows, sub-optimal solutions are usually derived to accommodate the real-time constraints of ATC systems. Nevertheless, we exhibit a significant advancement in this challenging class of problem. Also for the purpose of solving relatively large sized problems in practice, we develop and test suitable heuristic procedures. For the NAP, we propose a quasi-dynamic assignment scheme which is based on the incremental assignment technique. This quasi-dynamic assignment method assumes that the current aircraft route is influenced only by the previous aircraft assigned to the network. This simplified assumption obviates the need for iterative rerouting procedures to reach a pure equilibrium state which might not be achievable in practical taxiway operations. To evaluate the overall system, we develop a microscopic simulation model. The simulation model is designed to have the capability for reproducing not only the dynamic behavior of aircraft, but also incorporates communication activities between controllers and pilots. These activities are critical in ATC operations, and in some instances, might limit the capacity of the facility. Finally, using the developed simulation model named Virginia Tech Airport Simulation Model (VTASM) in concert with ASP and NAP, we compare the overall efficiencies of several control strategies, including that of the existing control system as well as of the proposed advanced control system.
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- Integrated Model to Plan Advanced Public Transportation Systems (1998)
Author: Dr. Chulho Bang
The primary objective of this study is to develop an integrated public transportation planning framework to evaluate and plan Advanced Public Transportation Systems (APTS). With this purpose, a systems approach point of view is adopted to study the influence of new APTS technology in supply and demand transit variables. In this project the Systems Dynamics methodology is adopted to track the dynamic behavior of model variables and feedback loops forming among them. The proposed framework is illustrated in a case study involving automated vehicle location systems (AVL) applied to a small transit community.
The proposed approach follows the same steps of the Systems Dynamics method; First, identify some key variables which are not only susceptive to AVL technology but also affect the supply-demand relationship of a bus transit environment. Second, trace and simplify the causal relationships of the variables considering impacts of facility supply changes to passenger demand responses and vice versa. To accomplish this, four detailed sub-models representing parts of the transit system are developed and combined under the Systems Dynamics methodology point of view. Theses Sub-models are: 1) demography, 2) urban transportation planning, 3) bus operations, and 4) evaluation. Finally, to validate the model procedure, the model is applied to a case study. This study attempts to encompass as many as possible factors around a bus transit system environment which can be impacted by new APTS technology to illustrate the use of the proposed framework. Some of these factors include: 1) Demographic characteristics; 2) urban or social activity of the study area and 3) changes to transportation facilities. The case study illustrates how the physical characteristics of the transit systems such as traffic demand, traffic conditions along the transit route, route layout, and bus performance can be affected by the new technology.
Since APTS impacts are time dependent a continuous multi-loop simulation technique is adopted to track dynamic changes of all model variables. The analysis of the transit system is carried over a 20-year life cycle to illustrate the long term dynamics of the feedback structures inherent in the model.
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- Modeling of Airport Operations Using an Object-Oriented Approach (1997)
Author: Dr. Caoyuan Zhong
This research develops an object-oriented approach to model airport ground network traffic
operations. A generic modeling library is developed as a tool kit to model the basic traffic
operations in the airfield using an object-oriented approach. The proposed generic modeling
library for airfield operations is a collection of predefined abstract components implemented
in the Java object-oriented programming language. Classes are defined and used as the basic
components in a variety of airfield operation modeling, simulations, and optimizations.The generic airport modeling framework consists of a set the components that are necessary
for modeling the basic activities of airfield traffic operations. By using the multi-threading
techniques, components are integrated into the proposed modeling framework. Unlike traditional
sequential simulation model, this framework organizes simulation activities into four
major groups which are: flight schedule, aircraft movement, time, and animation. Instead of
using built-in control logic, the framework adapts an open system policy which gives the
flexibility to the end users to incorporate the user-preferred control logic into the end models.
Another purpose in this research is to provide a future mechanism to study airfield
ground traffic automated control systems with Just-In-Time forecasting and model system
performance in a real-time ATC environment. The proposed generic library could be implemented
into a Internet/intranet ready application which can query real time information and provide real time advice to pilots and air traffic controllers. This study is one of a few current
research projects that are of using multiple threading technique to study traffic operation
The proposed generic library is originally implemented with C++ and, in the final stage, with
Java, a truly cross-platform object-oriented language. Application written in Java can run on
most of the mainstream computer operating systems without modifications. Although the
proposed library is for airfield traffic control system, it could also be extended into air traffic
control system as well as advanced transportation system.
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- Integrated Simulation and Optimization in Airfield Networks (1995)
Author: Dr. Xiaoling Gu
This dissertation is concerned with airport ground network operations and runway design problems. An aircraft landing simulation model is proposed to predict landing aircraft operations in airfield networks considering gate location and taxiway network information. By using the integrated simulation results, an improved dynamic programming model is established to solve the runway exit location problem.
The simulation model uses aircraft kinematic functions coupled with individual parameters to describe the landing process. A multiobjective optimization and a shortest path algorithm are used to predict the aircraft exit choice and expected taxiway path in a runway-taxiway network. By recognizing pilot motivation during the landing process, several influencing factors such as terminal location, traffic density, runway and weather conditions are considered in the aircraft landing simulation. Random variables such as aircraft approach speed, deceleration rate and runway exit speed are created to represent the stochastic aircraft landing behavior by using a Monte Carlo sampling technique. The model results, with some simplifications, could be used to solve runway exit location problem by providing the expected distribution of aircraft landing distances and aircraft runway occupancy times. With real-time input data the model could also provide information on air-craft exit choice, runway occupancy times and shortest taxiway path to an assigned terminal location for both the pilot and the air traffic controller in a ground traffic automatic control system.
The dynamic programming model is based on previous research results with substantial improvements. Instead of only focusing on the runway configuration, the proposed model recognizes airline terminal location as an influence factor in the runway exit location problem and introduces this influence in the optimization procedure.
An interactive computer program has been developed in C language on a RISC 6000 workstation to perform all these tasks. The choice of a workstation platform improves the performance of the dynamic programming algorithm when compared to personal computer solutions.
- Optimal Runway Exit Deisgn and Capacity Enhancement (1993)
Author: Dr. Byung Jong Kim
Relieving congestion of the air transportation networks requires several strategies to enhance the runway capacity. Among these strategies is reducing the runway occupancy time, a critical factor in affecting runway capacity.
In addressing the reduction of the runway occupancy time (ROT), complete information on the distribution of aircraft landing distance is required. A simulation model was built at Center for Transportation Research at Virginia Tech based on point mass kinematics in the flying phase over runway and the ground roll phase on runway to predict the landing roll distance and time to a specified exit speed. Many parameters were incorporated into the model, and then were calibrated using field data obtained from real operations.
The prediction of a nominal landing roll distance and time to decelerate to a specified exit speed is not sufficient for estimating ROT because the additional time to reach a designated exit should be taken into account. To compute the additional time, a braking adjustment scheme is selected from several alternative schemes. The combination of the selected braking adjustment scheme and the simulation model approximates very closely the observed ROT.
An optimization model is formulated to determine the exit locations so as to minimize the weighted average ROT of the defined aircraft mix. The major parameters for the model are the distribution of the landing roll distance to the specified exit speed and the information on the aircraft mix. The model is structured to address the problems of designing a new runway and of improving an existing runway.
A runway capacity model is used to convert the optimized ROT into capacity gains. Four scenarios are analyzed: one based on the present Air Traffic Control procedures, and three based on future developments. The capacity analysis reveals that the ROT does not affect the runway capacity for landing operations. However, the ROT is found as a critical factor for the runway capacity for mixed operations. Hence, the ROT should be optimized for the current system more crucially for the future developments. The capacity gains by optimizing the ROT under the current Air Traffic Control systems and standards are estimated 2 to 7 more operations per hour. These gains will increase to 20 more operations per hour in the future environment.
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- Combat Aircraft Scenario Tradeoff Models for Conceptual Design Evaluation (1988)
Author: Dr. Antonio Trani
The purpose of this research is to apply engineering-based knowledge to the field of combat aircraft survivability, and to create scenario-specific models in order to estimate the tradeoff between aircraft survivability and lethality metrics at the encounter and sortie levels. The development of scenario-specific models serves to identify and quantify technological changes that have leverage on the overall performance of the aircraft from a survivability point of view. Also, the models focus on the fighter aircraft susceptibility assessment and are capable of incorporating outputs from offline studies as inputs, such as in the area of vulnerability assessment where extensive data bases are available.
The mission scenario models are microscopic in nature and relate important conceptual aircraft design parameters such as thrust-to-mass ratio, wing loading, empty mass, maneuverability, etc. and operational parameters (e.g., weapon payload, range, loiter time, flight profiles, etc.) to the aircraft sortie survivability and lethality under various threat scenarios.
This research proposes a methodology to estimate survivability and lethality aircraft performance at the sortie level where aircraft parameters can be implemented into scenario specific models to assess their impact upon survivability-related metrics. While the project was conceived with naval aircraft in mind, the methodology, to the extent possible, is not to be aircraft-specific and thus could be applied to any particular design at the conceptual stage.