Guidelines for Cost-Effective Safety Treatments of Roadside Ditches (2021) / Chapter Skim
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From page 162... ...
150 CHAPTER 8. EVALUATION OF DITCH DESIGN VARIABLES PROBABILITY-WEIGHTED SIMULATION RESULTS AS A FUNCTION OF LATERAL DISTANCE The simulation outcomes were used to analyze each of the simulated roadway ditch configurations and the design variables of interest.
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151 represents one of the three performance measures (i.e., exceedance probability, rollover probability, or cumulative cost)
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152 For V-ditches, a significant percentage of rollovers occur on the backslope. As shown in Figure 8.4, the cumulative probability of rollover curves rises sharply in the backslope area.
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153 Figure 8.1. Exceedance probability of the extent of lateral encroachments: ditches with various BTWs, SHW = 6 ft, FSW = 8 ft, BSW = 8 ft.
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154 Figure 8.2. Exceedance probability of the extent of lateral encroachments: V-ditches with various SHWs, FSW = 8 ft, BSW = 8 ft.
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155 Figure 8.3. Cumulative rollover probability as a function of lateral distance: ditches with various BTWs, SHW = 6 ft, FSW = 8 ft, BSW = 8 ft.
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156 Figure 8.4. Cumulative rollover probability as function of lateral distance: V-ditches with various SHWs, FSW = 8 ft, BSW = 8 ft.
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157 Figure 8.5. Cumulative encroachment cost as a function of lateral distance: ditches with various BTWs, SHW = 6 ft, FSW = 8 ft, BSW = 8 ft.
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158 Figure 8.6. Cumulative encroachment cost as a function of lateral distance: V-ditches with various SHWs, FSW = 8 ft, BSW = 8 ft.
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159 GUIDELINE DEVELOPMENT The BCA method described in Chapter 5 was used to develop guidelines for the design of roadside ditches. The BCA method takes a large number of roadway, roadside, traffic, and encroachment variables into consideration.
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160 selection strategy. The preferred design envelope represents a set of ditch slope combinations for the default design condition beyond which the increase in rollover probability begins to accelerate rapidly as the slopes get steeper.
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161 Figure 8.8. An example cost contour map with equal contour interval.
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162 Figure 8.10. EAF moves preferred envelope for default design condition either inward or outward to stay neutral in crash cost.
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163 SHW: Shoulder width. FS: Foreslope ratio.
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164 The construction of the cost contour maps presented in Figure 8.17 and Figure 8.18 was based on the selection of 20 horizontal lines representing normalized cost levels from 0.1 to 2.0, with an equal cost increment of 0.1. To smooth out the contour lines, some interpolations and extrapolations were performed.
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165 Figure 8.11. Rollover probabilities for varying ditch bottom widths.
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166 Figure 8.12. Rollover probabilities for varying shoulder widths.
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167 Figure 8.13. Encroachment costs for varying ditch bottom widths.
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168 Figure 8.14. Encroachment costs for varying shoulder widths.
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169 Figure 8.15. Normalized encroachment costs for varying ditch bottom widths.
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170 Figure 8.16. Normalized encroachment costs for varying shoulder widths.
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171 Figure 8.17. Cost contour maps for varying ditch bottom widths.
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172 Figure 8.18. Cost contour maps for varying shoulder widths.
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173 Benefit-Cost Analysis Results and Preliminary Guideline Development This section presents BCA results for all ditch variables considered in the study. It further describes how the BCA results were used to determine the maximum risk level in terms of the expected encroachment cost for developing guidelines for various roadway-roadside configurations.
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174 Figure 8.19. NCPE contour maps for base configurations.
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175 cost increases as the FSW increases from 8 ft to 16 ft. It can be seen that when both the FS and BS are steep (the upper right quadrant)
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176 Figure 8.20. NCPE contour maps for varying ditch bottom widths.
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177 Figure 8.21. NCPE contour maps for varying shoulder widths.
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178 Figure 8.22. NCPE contour maps for varying vertical grades.
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179 Figure 8.23. NCPE contour maps for varying HCs.
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180 Figure 8.24. NCPE = 0.6 for varying BTW, FSW, and BSW.
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181 Figure 8.25. NCPE = 0.6 for varying HC, FSW, and BSW.
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182 Figure 8.26. Initial design envelopes for sites with ERs of 0.3, 0.5, 1.0, 1.5, and 3.0 per mi-yr-side.
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183 Figure 8.26. Initial design envelopes for sites with ERs of 0.3, 0.5, 1.0, 1.5, and 3.0 per mi-yr-side (continued)
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184 The same BCA and guideline development process described above for 2U55 was followed for three additional roadway design configurations: Two-lane undivided roadways with a 65-mph PSL (2U65)
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185 is not ideal for incorporation of design guidance into the AASHTO RDG. Further simplification of the preferred design envelopes into a more practical set of final recommended design guidelines is described in Chapter 10.
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186 Figure 8.27. NCPE contour map for simulated V-ditches with varying FSW and BSW: 4D65.
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187 Figure 8.28. Comparison of contour lines for NCPE=0.6 for four highway types -- PSL combinations.
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188 Figure 8.29. Design envelopes for relatively straight sections by ER and foreslope width.
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189 Figure 8.29. Design envelopes for relatively straight sections by ER and foreslope width (continued)
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190 Figure 8.29. Design envelopes for relatively straight sections by ER and foreslope width (continued)
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191 Figure 8.30. Design envelopes for sections with an HC of ≥3 degrees and <6 degrees by ER and foreslope width.
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192 Figure 8.30. Design envelopes for sections with an HC of ≥3 degrees and <6 degrees by ER and foreslope width (continued)
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193 Figure 8.30. Design envelopes for sections with an HC of ≥3 degrees and <6 degrees by ER and foreslope width (continued)
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194 Figure 8.31. Design envelopes for sections with an HC of ≥6 degrees by ER and foreslope width.
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195 Figure 8.31. Design envelopes for sections with an HC of ≥6 degrees by ER and foreslope width (continued)
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196 Figure 8.31. Design envelopes for sections with an HC of ≥6 degrees by ER and foreslope width (continued)
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