National Academies Press: OpenBook

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

Chapter: CHAPTER 3. SURVEY OF STATES

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Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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 3. SURVEY OF STATES." 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.
×
Page 51
Page 52
Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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.
×
Page 52
Page 53
Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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.
×
Page 53
Page 54
Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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.
×
Page 54
Page 55
Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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.
×
Page 55
Page 56
Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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.
×
Page 56
Page 57
Suggested Citation:"CHAPTER 3. SURVEY OF STATES." 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.
×
Page 57

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38 CHAPTER 3. SURVEY OF STATES INTRODUCTION TTI researchers developed a list of survey questions to identify innovative treatments of roadside ditches that may not have been previously documented. A web-based survey was created and posted through Survey Monkey. Survey participants were identified through FHWA, AASHTO Technical Committee on Roadside Safety, Transportation Research Board State Representatives Roster, and the project panel. A total of 42 (84%) states and four Canadian provinces (29%) responded to the survey, as shown in Figure 3.1. Figure 3.1. Responding U.S. states and Canadian provinces. - Responding Province

39 Determining Barrier Need and Slope Combinations How does your state determine if a barrier is warranted in front of a roadside ditch? Of the participants responding to the survey, 77% said they use the AASHTO RDG to determine if a barrier is warranted in front of a roadside ditch, while 36% use another source, primarily standards developed in their respective states. While most of the states indicated that their standards closely match those of the RDG, some rely on other factors, such as judgment based on the hazard presented, accident history, or ditch traversability. How does your state determine the foreslope and backslope combinations for the roadside ditches? Almost 64% of respondents use the RDG to determine the foreslope and backslope combinations for roadside ditches, while 49% said that they have developed their own criteria. It was found that 13% of the survey respondents answered both questions in that they used both the RDG and developed their own criteria. Ditch Geometry The participants were then asked questions related to the ditch geometry. A summary of the responses is presented next. The findings are also shown graphically in Figures 3.2 through 3.4. Ditch Shape The “V” is the most common ditch shape used, with 53% of respondents stating they use that shape more than 50% of the time; 38% stated they use a trapezoidal ditch more than 50% of the time. Ditch Symmetry Nonsymmetric ditches, where foreslope and backslope widths and slopes are not the same, are used more than 50% of the time by 65% of the respondents, while only 17% use a symmetric ditch more than 50% of the time. For those states/provinces using nonsymmetric ditches, the majority (75%) stated that the backslope is steeper than the foreslope.

40 Figure 3.2. Type of ditch typically used. Figure 3.3. Symmetry of the ditch.

41 Figure 3.4. Ditch configuration for typical nonsymmetric ditches. Ditch Dimensions Typical foreslope width: The most common foreslope is between 8–12 ft (2.4–3.6 m), followed by 16–20 ft (5–6 m). Typical bottom width: The most common bottom width is 2–6 ft (0.6–1.8 m). Typical backslope width: The most common backslope width is 8–12 ft (2.4–3.6 m), followed by 2–4 ft (0.6–1.2 m) and 16 ft (5 m) or over. Foreslope and Backslope The respondents were also asked to identify the most commonly used foreslope and backslope in their state. A short summary of the findings is presented below and graphically in Figures 3.5 and 3.6. Foreslope The most common foreslope is 6:1 (48% use it more than 50% of the time), and the least common is 10:1 or flatter (94% use it less than 25% of the time). Backslope Twenty-two percent of participants said they use a 3:1 backslope more than 50% of the time. An 8:1 or 10:1 or flatter backslope is used less than 25% of the time by 94% of participants.

42 Figure 3.5. Typically used foreslopes. Figure 3.6. Typically used backslopes. What does your state do to address ditch geometry in restrictive ROW widths? Three common techniques were used for addressing ditch geometry when ROW widths are restricted. The first is to steepen either the foreslope, backslope, or both. The second most

43 common method used is to install a closed drainage system to replace the ditch. Another common practice is placing a curb-and-gutter system. Slope Rounding Slope rounding is not a common practice among the states and provinces. Only 21% round the top break point, and 26% round the bottom of the ditch more than 50% of the time. Responses related to slope rounding are shown graphically in Figure 3.7. Figure 3.7. Usage of rounding as a mitigation strategy. Formula Used for Slope Rounding Thirty percent of the respondents stated that they use the formula found in the AASHTO Yellow Book for slope rounding. Formulas developed by the individual states and provinces are used 72% of the time. Surface Treatments The most common surface treatment used over 50% of the time is vegetation (67%), followed by turf (38%). The least used (less than 25% of the time) surface treatments are road base (94%), ditch paving (88%), and gravel (87%). This is shown graphically in Figure 3.8.

44 Figure 3.8. Types of surface treatments used. Pavement Location Use of ditch paving is shown graphically in Figure 3.9. Ditch paving is one of the least common surface treatments used on ditches, only 2% of the states using ditch paving pave the entire foreslope, entire backslope, or the full ditch more than 50% of the time. Pavement Width The most common width paved on the foreslope is 3 ft (1 m) and on the backslope between 3 and 4 ft (1 and 1.2 m). Innovative techniques your state uses or considers to reduce the crash risks associated with ditches. By far, the most innovative technique used by most states and provinces are rumble strips to keep drivers on the road. Another technique noted by several states is employing traversable drainage structures in lieu of ditches.

45 Figure 3.9. Use of ditch paving. CONCLUSIONS The researchers conducted a survey of state DOTs and Canadian provincial transportation agencies to identify innovative methods of treating roadside ditch hazards. The results of the survey were presented in this chapter. Although the results of the survey offered useful insights regarding different aspects of roadside ditch design features and related issues, they did not identify any innovative methods of mitigating hazards associated with motorists entering the ditches. Results indicated that some of the mitigation methods, such as slope rounding and surface treatment or paving, are used very infrequently. One of the most prevalently used mitigation method was the use of rumble strips near the edge of the roadway. Results of the survey were useful in selecting some of the design parameters used in the simulation matrix described in later chapters of this report.

Next: CHAPTER 4. ANALYSIS OF EXISTING CRASH DATA »
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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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