Browsing by Author "Dr. Nagui M. Rouphail, Committee Chair"

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Empirical Comparison of Simulation Models with Different Input Data Structures
(2002-11-18) Wan, Baohong; Dr. Stephen D. Roberts, Committee Member; Dr. Joseph E. Hummer, Committee Member; Dr. Nagui M. Rouphail, Committee Chair
This thesis focuses on an empirical comparison of CORSIM and Paramics, two commonly used traffic simulation models with different input data structures. The case comparison was executed between a field-validated CORSIM model and a fully calibrated Paramics network. These two models were constructed based on the same physical network dataset, which was originally created for the CORSIM simulation purposes. For those input data that were necessary for Paramics, but not available in this dataset, estimations were performed based on the known data and, sometimes, based on CORSIM default values. Of these the most important one was the Origin-Destination (OD) matrix. To enter traffic demand in Paramics, an OD matrix was derived using two different methods, namely a statistical fitting method and a stochastic assignment method. The feedback results from a Paramics test network showed that the stochastic assignment method was more effective in deriving a good OD solution. One straightforward finding of the comparison was that Paramics generated what appeared to be a larger percentage of unsuccessful runs than CORSIM. That was possibly because Paramics created more link flow fluctuations with the dynamic feedback traffic assignment algorithm; therefore, it had a higher chance of spillback or blockage for overloaded links or turn movements. A comparison of link flows in the two simulation models was executed based on the sample replications after excluding outliers. It displayed that there were some apparent link flow discrepancies between these two models. To ensure a meaningful comparison of other selected traffic performance measures, two critical corridors with minor vehicle flow discrepancies were selected as the comparison sites. By comparing the results on one corridor (NB LaSalle) , Paramics generated fewer vehicle trips and a higher vehicle travel speed, while on the other corridor (WB Ontario), the reverse occurred: although Paramics had fewer vehicle trips on that corridor, it still produced lower vehicle speeds than CORSIM. The research suggests that empirical comparisons of simulation models with different input data structures are feasible and informative for model validation and selection. Further, for the same traffic demand, Paramics generated traffic performance that is at variance with CORSIM?s when using dynamic feedback traffic assignment algorithm.
Impact of Lane Drops on Intersection Capacity
(2005-12-30) Lee, Jae-Joon; Dr. Billy M. Williams, Committee Member; Dr. Pamela J. Arroway, Committee Member; Dr. Joseph E. Hummer, Committee Member; Dr. Nagui M. Rouphail, Committee Chair
Lane drops downstream of signalized intersections are found on many urban and suburban streets and highways. The short lane is typically under-utilized at the intersection since drivers avoid using the short lane due to the potential for stressful merges downstream of the signal. This unconventional geometric configuration downstream of the lane drop intersection affects intersection capacity and operations. The current Highway Capacity Manual (HCM) does not take into account downstream lane drop conditions, thereby resulting in significant discrepancy between the HCM defaults and field observed lane utilization factors (LUF). The primary objectives of this research were to identify whether and by how much lane drops affect performance of actuated signalized intersections under various traffic demand levels and lane drop geometries. The research developed LUF prediction models by intersection type from data on traffic, signal and intersection geometry collected in the field. The developed models imply that the downstream lane length and traffic intensity are positively correlated with the LUF and that other geometric variables at the approach may also influence lane utilization. Also, a traffic simulation model was used to quantify the impact of lane drops on intersection operations with various traffic volumes and geometric conditions. Through the simulation, it was found that additional green time, which should reduce delay, was created because of the overloaded full lane at the approach, however; a higher probability of green time termination with maximum green time dominates the benefit of extra green and the approach gains significant extra delays. Based on the data obtained from the simulation, this research quantified the amount of additional delay for the subject movement and established delay adjustment factors due to account for lane drop effects. Findings from this research are expected to contribute to the assessment of lane drop intersection performance.
A Method for Prioritizing and Expanding Incident Management Assistance Patrols
(2004-06-07) Havel, Jason R.; Dr. Nagui M. Rouphail, Committee Chair; Dr. Joseph E. Hummer, Committee Member; Dr. Asad J. Khattak, Committee Member
Incident Management Assistance Patrols (IMAPs) enable smooth traffic flow by assisting stranded motorists and aiding in incident clearance. Currently, many large urban areas have IMAP services and the number of urban areas with IMAP programs continues to grow. The expansion of these programs is largely due to the successes of existing programs in the areas of traffic and incident management. However, the decision on where to expand the patrols is becoming ever more difficult as existing criteria typically suggest the high-priority areas that already have IMAP service; furthermore possible benefits of the service are often indistinguishable on lower-priority facilities. This project develops a new approach for identifying locations that can benefit from IMAP service in North Carolina through the use of expanded analyses criteria. Analysis of three incident/crash indices, desired IMAP coverage intensity, and delay estimations are combined to create a comprehensive methodology to evaluate and rank possible IMAP expansion sites. The results of this research were integrated into a decision-support software tool that allows easy assessment of candidate expansion site using planning and operational methods by comparing performance measures between sites, modeling the effects of IMAP on delay, and estimating potential benefits and costs impacts. Through the use of this tool, decision-makers can quickly and easily compare the needs of candidate sites to make an informed, cost-effective decision as to where to provide expanded IMAP service.
Methods for Calibrating and Validating Stochastic Microsimulation Traffic Models
(2003-12-08) Siddiqui, Nasir Uddin; Dr. Nagui M. Rouphail, Committee Chair; Dr. Joseph E. Hummer, Committee Member; Dr. John R. Stone, Committee Member
The purpose of this research was to propose a multistage framework for the calibration and validation of the traffic simulation models and present results of a calibration and validation experience using CORSIM model for a network of urban streets. The study proposed a series of logical, sequential steps for the calibration and validation of micro-simulation traffic models. The test bed used for the study is an important network of traffic signals in the city of Chicago, Illinois. The internal network consisting of twelve nodes at the core of the network served as the main focus of the calibration and validation experience for this study. Base data was collected using video and manual counts for extended AM and PM peak periods. Two methods for determining the number of model repetitions were proposed: a) use of statistical formula based on desired confidence interval and degree of confidence, and b) model-based sensitivity test which examines the number of outlier runs and the variability (distribution) in the model output from running sets of 25, 50, and 100 model runs. The study showed that both methods compliment each other in arriving at the required number of model repetitions. Automation processes using the REXX code was used for extracting the required model outputs and perform analysis of repetitive and multiple model runs during the calibration and validation processes. The calibration strategy adopted for the test network consisted of four distinct stages: a) error checking, b) calibration of input parameters for capacity and demand (throughput comparisons), c) model tuning (link attributes), and d) demand adjustment. The study showed that the concept of split links in modeling long term blockages by curb side parked vehicles proved to be more useful as compared to the NETSIM record types for long term events and parking activity. The study also showed the use of 'In' and 'Out' throughput volumes as an efficient and effective tool in calibration of micro-simulation models for urban street networks. Outputs from the calibrated and 'tuned' model for 100 replications were used in the model validation for test network. The research demonstrated the use of the mean stop time per vehicle and its modified form — the mean stop time per stopped vehicle as effective and efficient measures for use in validation of micro-simulation models. Between the mean stop time per vehicle and the mean stop time per stopped vehicle, the later proved to be more useful in the validation process primarily because it eliminates the difference between the model and the real-world which is purely on the basis of difference in the values of percent stops counts. For the test network, nine alternative scenarios were used for the model validation criteria in terms of the level of significance and the proportion of links using two-sample t-test. The study showed that the answer to the question whether the model is valid is dependant upon the satisfaction of the pre-defined criteria. The answer changes as the pre-defined validation criteria change. The key contribution of this research is the development of a multistage methodological framework for calibration and validation of micro-simulation traffic models. The methodology is quick to set up and implement on traffic networks and can be used beneficially by future analysts and researchers.
School Transportation Mode Choice and Potential Impacts of Automated Vehicle Location Technology
(2003-07-18) Rhoulac, Tori D.; Dr. John F. Monahan, Committee Member; Dr. John R. Stone, Committee Member; Dr. Joseph E. Hummer, Committee Member; Dr. Nagui M. Rouphail, Committee Chair
School-related traffic congestion is a large problem in North Carolina and throughout the nation. North Carolina Department of Transportation works with schools and municipalities to better design and retrofit campuses to accommodate the vehicular volumes that often queue onto adjacent streets and cause long delays. The large number of school trips made by automobile not only cause traffic problems, but safety problems as well, made evident by injury and fatality statistics. The goal of this research was to ultimately enhance student safety and reduce school-related traffic problems by gaining a better understanding of the household attributes and behaviors that influence school transportation mode choice in order to identify problems and prioritize solutions for school transportation, including school bus service improvements through automated vehicle location (AVL). The primary research objective was to calibrate a school transportation mode choice model for a selected North Carolina school district. Mode choice models were developed based on factors exhibiting statistical significance in estimating the choice of automobile or school bus for the morning and afternoon school trips of children in kindergarten through eighth grade in the Wake County Public School System. The variable expressing the convenience of the school bus service for a household based on parent work schedules, perceived problems, and AVL improvements was the most influential. Model transferability tests suggest that the models can be used statewide in schools or school districts where actual automobile usage ranges from about 30 to 55% of all school trips in the morning and 15 to 40% in the afternoon. The significant contribution of this research is the application of mode choice probability modeling to the grade school population. An assessment of the potential for AVL technologies to prompt a modal shift to the school bus was also included. Results suggest that 38% of students currently using the automobile for their morning school trips may shift to the school bus, indicating potential for a 16% overall modal shift to the school bus with the addition of AVL tracking and paging technologies.
Sustainable Service Rate Analysis at Signalized Intersections with Short Left Turn Pockets Using Macroscopic Simulation
(2010-04-20) Reynolds, William Leonard; Dr. Billy M. Williams, Committee Co-Chair; Dr. Nagui M. Rouphail, Committee Chair; Dr. George F. List, Committee Member
A macroscopic simulation tool is developed and tested in order to quantify the effects of short turn pockets on the sustainable service rate of a signalized intersection. Unlike the theoretical signal capacity, the sustainable service rate includes queue interaction effects and is thus influenced by blockage and spillback at the entrance to a short turn pocket. Previous research on the topic has focused either on the probability of spillback from a short turn pocket or the operation of a system with a single approach lane. No macroscopic model currently available has the ability to analyze throughput reductions due to short turn pocket effects on a multilane approach. The model described herein utilizes a series of flow and density restrictions on cells of varying sizes on the approach to the intersection. Results indicate sensitivity of the model to turn pocket spillback, blockage, saturation flow rate, pocket length, lane utilization, phase sequence, phase overlap, permitted phasing, and time-dependent demand. A phase optimization procedure is also described to help efficiently allocate green time for a given set of turn pocket lengths and turn movement percentages. Outputs from the model compare favorably to results generated using microsimulation software, and recommendations are made regarding additional model enhancements and testing needs.