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

Chapter: CHAPTER 10. GUIDELINE DEVELOPMENT

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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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Suggested Citation:"CHAPTER 10. GUIDELINE DEVELOPMENT." 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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215 CHAPTER 10. GUIDELINE DEVELOPMENT INTRODUCTION New findings emanating from this research are presented in entirety in the graphs shown in Figures 8.29 through 8.31 of Chapter 8. These 54 graphs show interdependence of all the variables considered by the research team for the different HCs and ERs considered. Although these 54 graphs can be used to design a roadside ditch, the research team realizes the complexity of using these graphs. For this reason, the research team extended significant effort in reducing the complexity of these 54 graphs, condensing and simplifying the information to arrive at easily usable guidelines for roadside ditch design. The process of simplifying and condensing these graphs into a final set of proposed guidelines is described in the sections to follow. Appendix D presents standalone guidelines and examples for their use that can be included in the RDG or other similar design documents. It should be noted that the simplification process described below required that some fidelity of the full findings of this research would be lost. To make the guidelines easily implementable, this simplification was considered a necessary task. However, even with the simplification, the proposed final guidelines presented in this chapter 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. In addition, changes or modifications in the foreslope, backslope, ditch width, and ditch material lining all have an effect on the hydraulic performance of the ditch (open-channel) drainage system. When evaluating the safety performance of ditches and channels, the designer must also evaluate the effect changes in geometry and lining will have on the mechanical performance of the drainage system. The guidelines proposed do not modify or change the need for the designer to employ traditional hydraulic design methods and considerations. GUIDELINE DEVELOPMENT PROCESS The proposed final design guidelines are comprised of six graphs, reduced from the 54 graphs in Figures 8.29 through 8.31 in Chapter 8. As presented in Chapter 8, the 54 graphs were comprised of three groups of 18 graphs, each group representing a different HC range. The three HC ranges were (a) HC of less than 3 degrees, (b) HC of greater than 3 degrees but less than 6 degrees, and (c) HC of greater than 6 degrees. For each HC range, the 18 graphs were further grouped into three groups of six graphs based on the bottom width of the ditch. The three ditch bottom width ranges were (1) bottom width of 0 ft to 4 ft, (2) bottom width of greater than 4ft to 8 ft, and (3) bottom width of greater than 8 ft to 12 ft. For each of these bottom width ranges, six graphs were presented for ERs that ranged from 0.75 to 4.50. As shown in Figure 10.1, each of the 54 graphs presents a preferred design envelope for a specific HC range, ditch BTW range, and an ER. In Figure 10.1, the graph shown is for an HC less than 3 degrees, BTW of 0–4 ft, and ER of 0.75. The foreslope of the ditch has been plotted versus the backslope. The graph contains five curves, each representing an FSW of 4 ft, 8 ft, 12 ft, 16 ft, and 20 ft, respectively.

216 Figure 10.1. Preferred design envelope graph for HC of less than 3 degrees, ditch bottom width of 0 to 4 ft, and ER of 0.75. As a first step in condensing the guidelines, the research team selected a single curve to represent all of the five foreslope widths. Responses obtained from the survey of state agencies conducted in the beginning of the project indicated that the 12-ft foreslope width was the most common ditch configuration. For this reason, only the curve representing the 12-ft wide foreslope was selected in all of the 54 graphs. This simplified the foreslope versus backslope preferred performance envelope shown in Figure 10.1 to the one shown in Figure 10.2. This process was carried out for all 54 graphs. Next, the researchers combined the six curves for each HC range and ditch bottom width, as shown in Figure 10.3. This resulted in one graph with the six ERs curves, as shown in Figure 10.3. Following this process, the number of graphs was reduced to nine. Further simplification of the guidelines was achieved by reducing the number of ditch bottom width ranges to two instead of three. As described earlier, the detailed performance envelopes presented in Chapter 8 were divided into three ditch bottom width ranges of 0 ft to 4 ft, greater than 4 ft to 8 ft, and greater than 8 ft to 12 ft. In the simplification process, the middle range for the ditch bottom width was merged into the other two width ranges—in other words, the results of the 0 ft to 4 ft ditch bottom width were considered applicable to a range of 0 ft to 6 ft. Similarly, the results of the greater than 8 ft to 12 ft bottom width were considered applicable to a ditch bottom width of greater than 6 ft. This simplification resulted in six graphs that constitute the final proposed roadside ditch design guidelines. These graphs are presented and discussed in the following section.

217 Figure 10.2. Simplified design envelope graph containing a single curve that represents all the foreslope widths. Figure 10.3. Curves of various ERs combined in a single graph. 0 0.1 0.2 0.3 0.4 0.5 0 0.2 0.4 0.6 Ba ck slo pe R at io Foreslope Ratio HC<3 ° BTW=[0,4)ft ER=0.75 Enc/mys 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Ba ck slo pe R at io Foreslope Ratio HC<3 °BTW=[0,4)ft

218 PROPOSED DESIGN GUIDELINES FOR ROADSIDE DITCH DESIGN The roadside ditch design guidelines are presented in Figure 10.4 through Figure 10.9. Following is a list of variables used in the guidelines: HC—Horizontal curvature (degrees). ER—Encroachment rate (encroachments/mile/year). BTW—Ditch bottom width (ft). Foreslope—Slope of the roadside ditch before ditch bottom. Backslope—Slope of the roadside ditch after ditch bottom. VG—Vertical grade (percent) The guidelines are grouped into three HC ranges of (a) less than 3 degrees, (b) greater than or equal to 3 degrees but less than 6 degrees, and (c) equal to or greater than 6 degrees. For each HC, the guidelines are further grouped into two ditch bottom width ranges of less than 6 ft and equal to or greater than 6 ft. In each of the figures, preferred design threshold curves for combinations of foreslopes and backslopes are presented. A total of six curves representing six ERs are plotted on each graph. To use the guidelines, a user first needs to determine the appropriate baseline ER. This determination incorporates the AADT of the roadway and is adjusted based on the HC. For a given roadway type (divided or undivided) and AADT, the designer uses Figure 10.10 to determine the appropriate baseline ER. The graph of the ERs versus AADT in Figure 10.10 has been developed for use in the most recent update to the RSAP program (63, 64). It was considered suitable for use in applying the roadside guidelines. The baseline encroachment rate for divided highways accounts for right side departures in the primary direction of travel. The baseline encroachment rate for undivided highways includes both primary right and opposite left side departures across the right side edge. The baseline encroachment rate relationships in Figure 10.10 have been developed for AADT of the primary side of the road. Thus, a bidirectional AADT should be divided by two, or the appropriate AADT for the primary direction of interest should be used if the AADT for the two directions are not equal. If there is an HC on the roadway, the designer will need to adjust the baseline ER for this curvature for the direction of interest. Carrigan et al. developed adjustment factors for the HCs that were considered suitable for use in applying the ditch design guidelines (63, 64). These adjustment factors are presented in Table 10.1 for divided and undivided roadways. Interpolation can be used to determine adjustment factors for HC values not listed in the table. The adjustment factor is multiplied with the baseline ER to obtain an adjusted ER that can be used for further applying the guidelines. If the roadway has a vertical grade, the baseline ER is adjusted for the vertical grade in the direction of interest. Table 10.2 presents adjustment factors for vertical grades for divided and undivided roadways. These were also developed by Carrigan et al. and were considered suitable for use with the ditch design guidelines (63, 64). By multiplying the adjustment factor with the baseline ER, an adjusted ER is obtained for further used in applying the guidelines.

219 If the roadway has both HC and vertical grade, adjustment factors are determined for both and each is multiplied by the previously determined baseline ER rate for use in the ditch design guidelines. After obtaining the appropriate ER, the designer determines which of the six figures (from Figure 10.4 through Figure 10.9) applies to the roadside ditch being evaluated. Depending on the HC and the ditch BTW, the designer selects the appropriate figure to use. After this, the designer plots the point representing the foreslope and backslope combination of the ditch being evaluated. If this point falls on or beneath the curve representing the ER curve, the foreslope and backslope combination is acceptable. If it falls outside of the curve for the applicable ER, the ditch configuration being considered is not within the recommended thresholds of the guidelines and should be adjusted by reconfiguring the slopes and/or changing the ditch bottom width. If the roadside ditch configuration cannot meet the proposed guidelines, the following guidance from RDG section 3.2.4 is suggested: “Channel sections that fall outside the shaded (acceptable) region are considered less desirable and their use should be limited where high-angle encroachments can be expected, such as outside of relatively sharp curves. Channel sections outside the shaded region may be acceptable for projects having one or more of the following characteristics: restrictive right-of-way environmental constraints; rugged terrain: resurfacing restoration, or rehabilitation (3R) projects; low-volume or low-speed roads and streets, particularly if the channel bottom and backslope are free of any fixed objects or located beyond suggested clear-zone distance. If practical, drainage channels with cross-sections outside the shaded regions and located in vulnerable areas may be reshaped and converted to a closed system (culvert or pipe) or, in some cases, shielded by a traffic barrier.”

220 Figure 10.4. Roadside ditch design guidelines for ditch bottom width of less than 6 ft and HC of less than 3 degrees. 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 a1 : b1 a 2 : b 2 Ba ck slo pe R at io = a 2 / b 2 Foreslope Ratio = a1 / b1 BTW < 6 ft, HC < 3 degrees 1:10 1:8 1:6 1:5 1:4 1:3 1:2 | | | | | | -1:8 -1:6 1:2 -1:3 -1:4 -1:5 -1:10

221 Figure 10.5. Roadside ditch design guidelines for ditch bottom width of greater than or equal to 6 ft and HC of less than 3 degrees. 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Ba ck slo pe R at io = a 2 / b 2 Foreslope Ratio = a1 / b1 BTW ≥ 6ft, HC < 3 degrees 1:10 1:8 1:6 1:5 1:4 1:3 1:21:2 1:3 1:5 1:10 1:8 1:6 1:4

222 Figure 10.6. Roadside ditch design guidelines for ditch bottom width of less than 6 ft and HC of greater than or equal to 3 degrees and less than 6 degrees. 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 a1 : b1 Ba ck slo pe R at io = a2 / b 2 Foreslope Ratio = a1 / b1 BTW < 6ft, 3 degrees ≤ HC < 6 degrees 1:10 1:8 1:6 1:5 1:4 1:3 1:2 | | | | | | 1:2 -1:3 -1:4 -1:10 -1:6 -1:8 a 2 : b 2 -1:5

223 Figure 10.7. Roadside ditch design guidelines for ditch bottom width of greater than or equal to 6 ft and HC greater than or equal to 3 degrees and less than 6 degrees. 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Ba ck slo pe R at io = a 2 / b 2 Foreslope Ratio = a1 / b1 BTW ≥ 6ft, 3 degrees ≤ HC < 6 degrees 1:10 1:8 1:6 1:5 1:4 1:3 1:2 | | | | | | a1 : b1 -1:5 -1:4 -1:6 -1:8 a 2 : b 2 -1:10 -1:3 1:2

224 Figure 10.8. Roadside ditch design guidelines for ditch bottom width of less than 6 ft and HC of greater than or equal to 6 degrees. 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 a1 : b1 a 2 : b 2 Ba ck slo pe R at io = a 2 / b 2 Foreslope Ratio = a1 / b1 BTW < 6 ft, HC ≥ 6 degrees 1:10 1:8 1:6 1:5 1:4 1:3 1:2 | | | | | | 1:2 -1:3 -1:5 -1:10 -1:8 -1:6 -1:4

225 Figure 10.9. Roadside ditch design guidelines for ditch bottom width of greater than or equal to 6 ft and HC of greater than or equal to 6 degrees. 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Ba ck slo pe R at io = a 2 / b 2 Foreslope Ratio = a1 / b1 BTW ≥ 6 ft, HC ≥ 6 degrees 1:10 1:8 1:6 1:5 1:4 1:3 1:2 | | | | | | -1:3 -1:5 -1:10 -1:8 -1:6 -1:4 a1 : b1 a 2 : b 2 1:2

226 Figure 10.10. ER based on AADT for divided and undivided highway types. Table 10.1. Adjustment Factors for ER Based on Horizontal Curvature Degree of Curvature* Rural Urban Undivided Divided Undivided Divided -25 3.11 1.00 2.07 1.00 -20 2.13 1.00 1.63 1.00 -15 1.46 1.00 1.28 1.00 -10 1.00 1.00 1.00 1.00 -5 1.00 1.00 1.00 1.00 0 1.00 1.00 1.00 1.00 5 1.00 1.00 1.00 1.00 10 1.00 1.00 1.00 1.00 15 1.11 1.00 1.03 1.00 20 1.23 1.00 1.07 1.00 25 1.36 1.00 1.10 1.00 *(-) curvature to left, (+) curvature to right 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000 En cr oa ch m en ts /M ile /Y ea r ( Pr im ar y D ir ec tio n, R ig ht S id e) Average Daily Traffic (vpd) Undivided Divided

227 Table 10.2. Adjust Factors for ER Based on Vertical Grade Grade* Rural Urban Undivide d Divided Undivided Divided -14 1.24 1.93 0.76 0.21 -13 1.21 1.81 0.78 0.24 -12 1.19 1.71 0.80 0.28 -11 1.17 1.61 0.82 0.32 -10 1.15 1.52 0.84 0.37 -9 1.12 1.43 0.86 0.42 -8 1.10 1.35 0.88 0.49 -7 1.08 1.27 0.91 0.56 -6 1.06 1.20 0.93 0.65 -5 1.04 1.13 0.95 0.75 -4 1.02 1.06 0.98 0.87 -3 1.00 1.00 1.00 1.00 0 1.00 1.00 1.00 1.00 3 1.00 1.00 1.00 1.00 4 1.01 1.05 0.97 0.85 5 1.02 1.10 0.94 0.72 6 1.03 1.16 0.91 0.61 7 1.04 1.22 0.89 0.51 8 1.05 1.28 0.86 0.43 9 1.06 1.34 0.83 0.37 10 1.08 1.41 0.81 0.31 11 1.09 1.48 0.78 0.26 12 1.10 1.56 0.76 0.22 13 1.11 1.64 0.74 0.19 14 1.12 1.72 0.72 0.16 *(-) vertical grade down, (+) vertical grade up

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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.

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