Guidelines for Cost-Effective Safety Treatments of Roadside Ditches (2021) / Chapter Skim
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197 CHAPTER 9. EVALUATION OF POTENTIAL MITIGATION METHODS INTRODUCTION Ditch-initiated crashes are currently costing American society about $6.6 billion annually (in 2010 dollars)
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198 Rounded slope break points can also be more expensive to maintain, particularly the toe of the slope. Maintaining a well-rounded toe may be difficult due to ditch erosion resulting from hydraulic flow.
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199 Table 9.1. AADT range for recommended slope rounding (20)
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200 One can see from this formulation that the lateral extent of optimum rounding is a function of shoulder slope, sideslope, and encroachment speed and angle. For example, given a 4% shoulder slope, a 4H:1V sideslope, and design encroachment conditions of 60 mph (97 km/h)
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201 Simulated Ditch Configurations As previously discussed, to evaluate the effect slope rounding has on vehicle stability, overturn and the benefit to cost, the two types of slope rounding treatments were simulated and studied: Constant slope rounding: 6 ft (3 ft along shoulder + 3 ft along foreslope)
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202 rounding scenario are 0.68, 1.14, 1.70, and 2.75, respectively. Note that the NCPE values for the 6:1-6:1 FS-BS combination with no-rounding treatment are also shown in each plot (in gray)
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203 NCPEno(FSW, BSW, FS, BS) is used to represent the NCPE value of a configuration with FSW, BSW, FS, and BS, and with the no-rounding treatment.
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204 The objective of the estimation procedure is to estimate the NCPE ratios, fcr(FSW, BSW, FS, BS)
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205 Summary for Guideline Development In general, both types of slope rounding treatments appear to reduce the encroachment cost in most of the design configurations simulated in this study (29 out of the 32 configurations simulated) , and the optimum rounding appears to perform better than the constant rounding for most of the simulated configurations.
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206 Figure 9.2. NCPE for simulated ditch configurations with and without slope rounding treatments: 4D65, BTW = 4 ft.
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207 Figure 9.3. NCPE contour maps constructed from simulated configurations with and without slope rounding treatments: 4D65, BTW = 4 ft.
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208 Figure 9.4. Contour lines for NCPE values of 0.2, 0.6, and 1.2 for ditches with and without slope rounding treatments: 4D65, BTW = 4 ft.
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209 Figure 9.5. NCPE contour maps constructed from simulated configurations with and without slope rounding treatments: 4D65, BTW = 0 ft.
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210 Figure 9.6. NCPE contour lines of 0.2, 0.6, and 1.2 for ditches with and without slope rounding treatments: 4D65, BTW = 0 ft.
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211 Figure 9.7. NCPE contour lines of 0.2, 0.6, and 1.2 for ditches with and without slope rounding treatments: 2U55, BTW = 4 ft.
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212 Figure 9.8. NCPE contour lines of 0.2, 0.6, and 1.2 for ditches with and without slope rounding treatments: 2U55, BTW = 0 ft.
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213 SURFACE TREATMENTS As determined from the survey of participating states, the most common surface treatment used is vegetation, followed by turf. The least used surface treatments are road base, ditch paving, and gravel.
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214 Figure 9.9. NCPE contour maps for varying surface treatments.
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