National Academies Press: OpenBook

Guidelines for Cost-Effective Safety Treatments of Roadside Ditches (2021)

Chapter: CHAPTER 11. SUMMARY AND CONCLUSIONS

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Suggested Citation:"CHAPTER 11. SUMMARY AND CONCLUSIONS." National Academies of Sciences, Engineering, and Medicine. 2021. Guidelines for Cost-Effective Safety Treatments of Roadside Ditches. Washington, DC: The National Academies Press. doi: 10.17226/26127.
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Page 240
Page 241
Suggested Citation:"CHAPTER 11. SUMMARY AND CONCLUSIONS." National Academies of Sciences, Engineering, and Medicine. 2021. Guidelines for Cost-Effective Safety Treatments of Roadside Ditches. Washington, DC: The National Academies Press. doi: 10.17226/26127.
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Page 241

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228 CHAPTER 11. SUMMARY AND CONCLUSIONS This research project developed guidelines for designing cost-effective roadside ditches that mitigate the severity of crashes on slopes. The research team gathered data on current practices for roadside ditch designs by conducting a survey of state transportation agencies. Information about most commonly used ditch configurations and any innovative ditch hazard mitigation methods were collected. Results of this survey were used to guide the selection of various ditch design parameters for simulation under this project. The survey did not identify new ditch hazard mitigation methods; however, information was collected for design and usage of known methods, such as slope rounding and surface treatment. The research team also analyzed several existing crash databases to identify 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. Crash data analysis was also used to extract potentially useful data for supporting the conduct of a BCA, which was then used in the process of developing the 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 devised a BCA method suitable for developing the design guidelines. The researchers developed procedures for carrying out the BCA method to demonstrate the feasibility of the method. Available data needed to support the analysis were also identified and obtained. Details of the BCA method used are presented in Chapter 5. To evaluate the influence of various roadside ditch design parameters, vehicle designs, and driver inputs on vehicle stability on slopes, the research team carried out an extensive simulation effort. The research team evaluated various simulation tools for their suitability for this project. Multi-rigid body vehicle dynamics code was eventually selected. The researchers also incorporated several features, such as the application of soil-furrowing forces for side slipping vehicles, vehicle body-to-terrain contact, and more, into the commercially available vehicle dynamics simulation code. Details of simulation tool selection and various features that were built into the vehicle dynamics model are presented in Chapter 6. To efficiently perform a large number of simulations, the researchers also developed a simulation management software that generated the large number of simulation inputs required, performed simulations in batch mode, and extracted and organized simulation outcomes in a prescribed format for further evaluation in the statistical analyses. The researchers also developed simulation models of the various vehicles used in the simulation matrix and conducted several small-scale sensitivity studies to determine suitable values for some of the simulation parameters. Details of simulation management software and the vehicle models developed are presented in Chapter 6. To evaluate the effects of combinations of ditch design parameters, encroachments conditions, and vehicle and driver inputs on vehicle stability, the researchers carried out an extensive simulation effort that was comprised of 633,760 simulation cases (Chapter 7). After performing simulations with the selected ditch configurations and encroachment conditions, the research team performed an extensive evaluation of the ditch design variables (Chapter 7). This effort involved performing some exploratory analyses, next 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

229 the acceptability of various ditch design combinations and to arrive at the final design guidelines. Details of the exploratory analyses, development of cost contour maps, and guideline development process are presented in Chapter 8. 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 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. The researchers found that while slope rounding treatments do appear to reduce the encroachment cost in most of the design configurations, the reduction in cost is not very significant. The researchers also determined that using ditch surface treatments for mitigating rollover has a very small effect. In some ditch configurations analyzed, there was an actual increase in the encroachment cost compared to the untreated ditch surface. Based on these findings, slope rounding and surface treatment were not considered suitable ditch hazard mitigation methods (Chapter 9). The simulation analyses and BCA performed in this project resulted in 54 graphs showing the interdependence of all the variables considered by the research team (Chapter 8). Although these graphs can be used to design a roadside ditch, the research team extended significant effort in reducing the size and complexity of these graphs. This processes involved condensing and simplifying the information to arrive at easily usable guidelines. Even though the simplification process contributed to some loss of fidelity of the full findings of this research, it was considered a necessary sacrifice to achieve overall success in the implementation of the research’s findings. Even with the simplification, however, the proposed final guidelines retain sufficient fidelity to provide a ditch designer with many options depending on the site and traffic. The final proposed guidelines are therefore a significant enhancement to the current design guide and fully meet the objectives of this research. The guidelines are presented in Chapter 8 and also in Appendix D as a standalone section with examples of its use. Results of this research can be implemented by including Appendix D in RDG.

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Run-off-road traffic crashes account for almost one-third of the deaths and serious injuries each year on U.S. highways.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 296: Guidelines for Cost-Effective Safety Treatments of Roadside Ditches provides new proposed design guidance for the configuration of ditches adjacent to the roadway.

A supplemental PowerPoint Summary is available.

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