Development of Safety Performance-Based Guidelines for the Roadside Design Guide (2022) / Chapter Skim
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Pages 144-163
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From page 144... ...
144 The following sections present the results of converting previously developed risk-based guidance for the RDG into the format recommended in Chapter 3: Roadside Risk Design Methodology. 4.1 Median Barrier Selection Median barrier selection guidelines were developed using the method outlined in Chapter 3: Roadside Risk Design Methodology, by Carrigan and Ray (Carrigan and Ray 2022)
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From page 145... ...
Results 145 • The shielding barrier crash severity values found in Table 45 for cable (0.005) , metal beam (0.0094)
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From page 146... ...
146 Development of Safety Performance-Based Guidelines for the Roadside Design Guide the section shown as concrete/metal beam/cable in Figure 39 (e.g., 30-foot-wide median with an AADT of 90,000 veh/day) , a cable, metal beam, or concrete barrier would reduce the risk of a fatal or serious injury crash when compared to an unprotected median.
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From page 147... ...
Results 147 barrier. For a 30-foot-wide median, the relative risk of a cable median barrier with respect to no median barrier is one for an AADT of 25,000, 0.75 for an AADT of 40,000, and 0.5 for an AADT of 60,000 veh/day.
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From page 148... ...
148 Development of Safety Performance-Based Guidelines for the Roadside Design Guide For the 30-foot median highway discussed here, a high-tension cable median barrier will not become cost-beneficial until the traffic volume exceeds 40,000 veh/day as shown in Table 49. If the traffic volume increases to 60,000 veh/day on this same highway ORAlt/Null increases to 0.0051 and the BCR will increase to a value of just over 2.5.
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From page 149... ...
Bi-Directional AADT (veh/day) Traversable Median Width (ft)
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150 Development of Safety Performance-Based Guidelines for the Roadside Design Guide 4.2 Relocating Narrow Fixed Objects The second 1967 Yellow Book strategy is to "relocate the hazard to a point where it is less likely to be struck" (AASHTO 1974b)
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From page 151... ...
Results 151 4.3 Shielding Object-Free Sloped Terrain A number of researchers over the past several decades have noted that for slopes that are free of all other features (e.g., trees, poles, etc.) , shielding the slope with a w-beam guardrail does not reduce the risk of observing a fatal or serious injury crash (Glennon and Tamburri 1967; Zegeer et al.
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From page 152... ...
152 Development of Safety Performance-Based Guidelines for the Roadside Design Guide where terms are as previously defined for Equation 2 in Section 3.3.1: Method, with the subscripts indicating the following: BAR = Shielding longitudinal barrier. TER = The roadside terrain.
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From page 153... ...
Results 153 Full details are in NCHRP Research Report 996 and the derivation and full equations are shown in Appendix B.4 (Carrigan and Ray 2022)
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From page 154... ...
154 Development of Safety Performance-Based Guidelines for the Roadside Design Guide The NCHRP 17-43 encroachment data indicates that the 15th percentile encroachment angle is approximately five degrees, the 50th percentile encroachment angle is 11 degrees, and the 85th percentile encroachment angle is 22 degrees (from NCHRP Project 17-88, "Roadside Encroachment Database Development and Analysis")
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From page 155... ...
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 ) tf( tcejb O dexiF fo htdi W [W B FO W F FO ]
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From page 156... ...
156 Development of Safety Performance-Based Guidelines for the Roadside Design Guide same way as was used in the previous example illustrated by Figure 41. If the spacing between the narrowly fixed objects is large enough, each feature should be considered a separate individual feature.
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Results 157 if it is closer than 32 feet to the edge of lane for a relative risk of 0.75. Similarly, a 20-foot-wide (i.e., parallel to the road)
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158 Development of Safety Performance-Based Guidelines for the Roadside Design Guide 25% fewer KA crashes are expected for the shielded location. It was found that relative risks of 0.50 were seldom possible except for very long and very wide features.
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Results 159 area. Penetrating the barrier may limit or impose severe limitations on the regional transportation network (i.e., interstates, rail, etc.)
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160 Development of Safety Performance-Based Guidelines for the Roadside Design Guide To select the appropriate bridge railing based on highway and traffic conditions perform the following steps -- 1. Traffic Conditions: Determine the anticipated mid-life AADT volume and percentage of trucks (PT)
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From page 161... ...
Results 161 vii. The capacity of the bridge deck may limit the choices available for higher test level bridge railings on rehabilitation projects.
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From page 162... ...
162 Development of Safety Performance-Based Guidelines for the Roadside Design Guide 0 1 2 0 5 10 15 20 25 30 35 40 45 50 Percent Trucks Low Risk TL3 TL4 TL5 0 1 2 0 5 10 15 20 25 30 35 40 45 50 M od ifi ed E nc ro ac hm en ts (E nc r/m ile /y ea r)
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From page 163... ...
Results 163 Figure 45. Minimum horizontal curve radius based on barrier obstruction to the stopping sight distance compared to AASHTO Exhibit 3-14 (AASHTO 2011a)
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