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1 SUMMARY This report provides new proposed design guidance for the configuration of ditches adjacent to the roadway. The objective of this project was to develop guidelines for cost- effective design of roadside ditches that mitigate the severity of crashes on slopes by taking into consideration a more recent vehicle fleet. To meet this objective, the research team performed a detailed survey of state transportation agencies and a Canadian province to gather data on current practices for roadside ditch design. The survey was comprised of questions regarding the types of most common ditch configurations in the state, use of slope rounding, surface treatment, or other innovative practices for mitigating slope-related crashes. Although the survey did not identify a new mitigation method 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. An additional objective was to extract potentially useful data to support conducting a benefit-cost analysis (BCA) that would assist in developing ditch design guidelines. 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 identified and obtained. To evaluate the effect of a combination of ditch parameters, encroachments conditions, and vehicle and driver inputs on vehicle stability, an extensive simulation effort was carried out. The research team evaluated various simulation tools for their suitability to this project. In addition, several features, such as application of soil-furrowing forces for side slipping vehicles, vehicle body-to-terrain contact, and the like, were incorporated into 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. 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 performing some exploratory analyses, developing cost contour maps using the BCA model, and 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. 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 treatments were
2 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 understand if there was significant benefit to using them. Because of the large number of 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 both encompass key findings of the analyses and are simple enough for a ditch designer to use. The process and result of this project are proposed guidelines for use by designers to improve safety performance of roadway ditch designs. The proposed guidelines are in Appendix D of this report. In addition, the research team provided examples of how to use the guidelines for real-world problems.
