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36 CHAPTER 2. RESEARCH APPROACH The objective of this project was to develop guidelines for the cost-effective design of roadside ditches to mitigate the severity of crashes on slopes and take into consideration a more recent vehicle fleet. To meet this objective, the research team carried out various tasks that are explained in detail in the following chapters of this report. The overall research approach taken by the research team, a higher-level summary of the various tasks, and how they are interconnected is presented in this brief chapter. At the start of the project, the research team carried out a detailed survey of the state transportation agencies to gather data on current practices for roadside ditch design. The survey was comprised of questions related to the types of most common ditch configurations in the state and the use of slope rounding, surface treatment, or other innovative practices for mitigating slope-related crashes. The details of this survey and its results are presented in Chapter 3 of this report. While the survey did not unfold a new mitigation method being used to reduce slope- related crashes, the results of the survey guided the research team in selecting values of some of the ditch design parameters for further evaluation through simulation analyses. In parallel to the survey, the research team evaluated and analyzed several existing crash databases. The objective was to identify any trends in the type and severity of ditch-related crashes and their relationship to ditch geometry, roadway characteristics, vehicle type, presence of appurtenances, and other relevant characteristics. Another objective was to extract potentially useful data for supporting the conduct of a BCA that would be used to develop ditch design guidelines. The details of the crash database evaluation and analyses are presented in Chapter 4 of this report. Based on the results of the initial literature review, survey of current practice, and analysis of existing crash data, the research team researched and subsequently devised a BCA method suitable for meeting the objectives of this project. Procedures to carry out the BCA method were developed to demonstrate feasibility, and the best available data to support the analysis were also identified and obtained. Details of the basic framework and concepts used, along with a detailed description of the BCA model, are presented in Chapter 5 of this report. To evaluate the effect of a combination of ditch parameters, encroachment conditions, and vehicle and driver inputs on vehicle stability, an extensive simulation effort was carried out in this project. Prior to embarking on the large simulation analysis, the research team evaluated various simulation tools for their suitability for this project. Additionally, several features such as application of soil-furrowing forces for side slipping vehicles, vehicle body-to-terrain contact, and others were incorporated in the simulation tools. An extensive simulation management software was developed to generate the large number of simulation inputs required, perform simulations in batch mode, and extract and organize simulation outcomes in a prescribed format for further evaluation in the statistical analyses. Simulation models of the various vehicles used in the project were also developed. Various small-scale sensitivity studies were conducted to determine suitable values for some of the simulation parameters. All of this upfront work leading up to the performance of the final simulation analyses is presented in Chapter 6 of this report. The final simulation matrix, which determined the simulations performed and analyzed under this project, is presented in Chapter 7. Some examples of the simulation cases and their results are also presented in this chapter.
37 After performing encroachment simulations with the selected ditch configurations and encroachment conditions, the research team performed an extensive evaluation of the ditch design variables. This effort involved (a) performing some exploratory analyses, (b) developing cost contour maps using the BCA model, and (c) eventually using these cost contour maps to determine the influence of the selected ditch design variables on crash severity. The cost contour maps were used to determine the acceptability of various ditch design combinations and to arrive at the final design guidelines. Evaluation of the ditch design variables, initial exploratory analyses, details of the development of cost contour maps, and their use in understanding the influence of various ditch design parameters is presented in detail in Chapter 8 of this report. In the beginning of the project, an emphasis was placed on evaluating various mitigation methods for reducing ditch-related crashes. Ditch rounding and ditch surface treatment were identified as potential mitigation methods, albeit the survey of states indicated minimal use of these strategies. A focused evaluation of these two potential methods was performed under this project to fully understand if there was significant benefit to using them. Details of this evaluation are presented in Chapter 9. Due to the large number of the ditch design parameters evaluated under this project, the researchers spent significant effort in evaluating the various trends identified from the BCA and in extracting final ditch design guidelines that encompass key findings of the analyses and are simple enough for a ditch designer to use. The process of arriving at the design guidelines and the final proposed guidelines are presented in Chapter 10 of this report. The proposed guidelines have also been presented as a standalone section in Appendix D of this report. The research team has also provided some particular examples of how to use the guidelines for real-world problems in Appendix D.
