Development of Safety Performance-Based Guidelines for the Roadside Design Guide (2022)

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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). They used a relative risk approach to compare the relative risk of shielding with a median barrier to leaving the median unshielded. A relative risk approach is appropriate for median barrier type selection and needs determination because both alternatives (i.e., shield with a median barrier and leave the median unshielded) have safety benefits. Median barriers can help minimize the number of vehicles that cross over the median, but there is some risk associated with striking the median barrier. On the other hand, an unshielded median provides space for the errant vehicle to come to rest but poses a risk of some vehicles crossing over the entire median. A median barrier should only be used when the risk of a KA ROR crash for the shielded median is less than the unshielded median as follows: IF ( ) ( )( ) ( ) ( ) ( ) ( ) > = + −  − • • • • • 1 OUTCOME OUTCOME P MW 2 P P MW P 1 THR P MW P 1 THR SHLD UNSHLD Y SEV BAR Y SEVCMC EOL Y SEVCMC EOL THEN Install a median barrier. ELSE Leave the median unshielded by a median barrier where terms are as previously defined for Equation 2 in Section 3.3.1: Method with the sub- scripts indicating the following: SHLD = The alternative with a median barrier. UNSHLD = The unshielded median. This is the Null alternative. BAR = Median barrier. CMC = Cross-median crash event. EOL = Edge of opposing lanes. The derivation of the equation is provided in Appendix B.1, and NCHRP Research Report 996 offers full details (Carrigan and Ray 2022). The three independent variables are median width (MW), bi-directional AADT volume, and one of three barrier types (e.g., cable, metal beam, or concrete). The remaining values are found from the following: • Values for the lateral extents of encroachment PY(MW) and PY(MW/2) are found in Table 41 for values between 4 and 100 feet. C H A P T E R 4 Results

Results 145   • The shielding barrier crash severity values found in Table 45 for cable (0.005), metal beam (0.0094), and concrete (0.0159) median barriers. • Harm accrues to all vehicles that interact with a longitudinal barrier (i.e., δBAR = 0). • The crash severity of a cross-median crash found in Table 45 (0.0451). • Harm accrues only to vehicles that strike another vehicle in the opposing lanes (i.e., δEOL = 1). Vehicles that cross the opposing lanes without interacting with another vehicle accrue no harm. • The barrier has a negligible probability of penetration by passenger vehicles (i.e., THRBAR = 0) since it is assumed the percentage of trucks is less than 5%. • The proportion of vehicles that pass all the way across the opposing lanes (THREOL) is found in Table 44 for AADTs between 1,000 and 12,000 veh/day. Figure 39 shows the resulting guidelines for median barrier need and type on controlled-access roadways with unobstructed medians (i.e., longitudinal barriers are not needed for shielding fixed objects like trees, utility poles, or bridge piers in the median). To determine the need for a median barrier, plot the point corresponding to the design year bi-directional traffic volume and median width in Figure 39. The area where this point plots indicates the type of median barrier most appropriate for the site and traffic conditions. For example, a cable median barrier would be most appropriate for a 30-foot-wide traversable median with an AADT of 40,000 veh/day as shown in Figure 39. On the other hand, a 30-foot-wide traversable median on a controlled- access highway with bi-directional traffic volume in the design year of only 10,000 veh/day is better left with no median barrier. A rigid concrete, metal beam, or cable median barrier would reduce the median crossover crash risk on a controlled-access highway with a 30-foot travers- able median and 95,000 veh/day as shown in Figure 39. The particular choice between concrete, metal beam, or cable in this situation would be made based on available deflection distance, cost, and other factors, but in this area, a median barrier of any type would reduce the risk compared to not having a median barrier. There are two areas located on the left side of Figure 39. In these areas, either cable or metal beam median barriers could reduce the risk of a fatal or serious injury crash compared to not having a median barrier, but median barriers in these areas may allow dynamic deflection of the barrier into the opposing lanes. If the design objective is to accommodate all the barrier deflec- tion within the median, cable median barriers should not be used in medians narrower than 16 feet, and metal beam barriers should not be used in medians narrower than 10 feet because they could deflect into the opposing lanes in a crash. If occasional deflection into the opposing lanes is an acceptable design result, cable or metal beam barriers could be used in these areas of Figure 39. Unlike previous median barrier guidance, the selection depends on the type of barrier con- sidered because the crash severity for each type of median barrier differs. The proportion of fatal and serious injury crashes for concrete, metal beam, and cable median barriers were shown earlier in Table 45 to be 0.0159, 0.0094, and 0.005, respectively. Since the relative risk compares the risk of crashes with median barriers to the no median barrier condition the choice of median barrier affects the ratio. Once the type of median barrier has been selected and the need established, the appropriate test level should be identified. Table 48 is used to select the appropriate median barrier test level as a function of the percentage of trucks in the traffic mix in the design year. For the example discussed above, a test level three cable median barrier would be appropriate for a 30-foot-wide median with an AADT of 40,000 veh/day and less than 10% trucks. If the percentage of trucks is 12%, a test level four cable barrier would be appropriate. If a test level four cable median barrier is not available, a test level three cable barrier should be used as neither metal beam nor concrete barriers would provide a lower risk than leaving the median unprotected for these parameters. If a traffic volume and traversable median width for a particular roadway were to plot within

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. If the percentage of trucks stands at 18% at this particular site, a test level five concrete barrier would be a good choice based on Figure 39 and Table 48. The foregoing analysis determined whether a median barrier resulted in fewer fatal and serious injury cross-median crashes than the no-median barrier (i.e., null) alternative. A high- way agency may base its median barrier selection and need policy simply on the relative risk as shown in Figure 39. The farther away the plotted point is from the line below it, the larger the relative risk reduction. Plotting on the line (e.g., a median width of 30 feet and AADT of 25,000 veh/day plots on the cable median barrier line in Figure 39) indicates a relative risk of one, meaning the median barrier is no worse but also no better than not having a median 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 4 10 16 22 28 34 40 46 52 58 64 70 76 82 88 94 100 Median Width (feet) Consider concrete, metal beam or cable Consider metal beam or cable Consider metal beam if deflection into opposing lanes is acceptable to user agency Consider cable Consider cable if deflection into opposing lanes is acceptable to user agency Median Barrier Not Normally Considered B i-d ire ct io na l A A D T (v eh /d ay ) Figure 39. Guidelines for median barrier type selection (Carrigan and Ray 2022). MASH Test Level Traffic Conditions 2 or higher Zero PT and PSL ≤ 45 mph 3 or higher 0 < PT ≤ 10 4 or higher 10 < PT ≤ 15 5 or higher ≥ 15 PT or a designated truck or hazardous material route Note: PT = percentage of trucks; PSL = posted speed limit. Table 48. Guidelines for selection of longitudinal barrier test level (Carrigan and Ray 2022).

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. A highway agency could establish a policy that cable median barriers are only installed when the risk is half that of the no median barrier alternative (i.e., the relative risk is 0.50). The choice for the appropriate relative risk is a policy decision the highway agency could make based on need, resources, or economics. The economic impact of the median barrier alternative decision can be assessed using benefit- cost (BCR), cost-effectiveness (ICER), or the IRR as discussed in Section 3.3.3: Segment Risk and Making Design Decisions. The effectiveness of the alternative measured is the difference in the annual frequency of fatal and serious injury for the design alternative being considered and the existing unshielded median (ORAlt/Null = OUTCOMENull − OUTCOMEAlt). Values for ORAlt/Null are tabulated for cable, metal beam, and concrete median barriers in Table 49. For example, a median barrier in a 30-foot-wide traversable median on a four-lane highway with 25,000 veh/day results in a higher frequency of KA crashes than the unshielded median so a median barrier should not be used as indicated in Table 49 by a blank cell. On the other hand, if a cable median barrier were installed on a four-lane divided highway with 30,000 veh/day and a 30-foot-wide median Table 49 indicates that 0.0005 fewer KA cross-median crashes can be expected annually. In this case, the cable median barrier is risk beneficial although it may or may not be cost-beneficial. The cable median barrier reduces the risk of a cross-median crash, but it remains to be seen if providing the median barrier will be a good return on the funds invested. The next step is to calculate the direct costs. The direct cost of the null alternative is assumed to be zero because it represents the already-existing traversable median. The direct cost of the median barrier being considered as an alternative is the direct cost of construction added to the present worth of the annual maintenance cost. For example, assume high-tension cable median barrier has a direct installation cost of $125,000 per mile of median and an annual maintenance cost of $2,500 per mile per year. Further, assume that no major earthwork is required before installing the median barrier, the design life is 30 years, and the rate of return is 2%. The present worth of the direct cost of installing the high-tension cable median barrier is therefore: + = + =• • •DC AP MC PA $125,000 0.0446 $2,500 $8,075Alt i,n Alt i,n For a highway with a 30-foot-wide traversable median and bi-direction design year traffic of 30,000 veh/day, the expected annual reduction in cross-median crashes (ORAlt/Null) is 0.0005, as shown in Table 49. Given a design year VSL of $12.3 million, CKA is 0.33, as described earlier in Section 3.3.3: Segment Risk and Making Design Decisions, and the direct costs above, the BCR can be calculated as follows: +       = +     =• • • • • • òBCR =OR C VSL DC AP MC 0.0005 0.33 12,300,000 125,000 0.0446 2,500 0.25 1Alt Null KA Alt i,n Alt A high-tension cable median barrier is not cost-beneficial on a 30-foot-wide median with 30,000 veh/day under these economic assumptions even though the cable median barrier does reduce the risk. A high-tension cable median barrier would have a BCR greater than one any- where in Table 49 with a value greater than 0.002 as follows: +    +    = • • • • BCR DC AP MC C VSL =OR 1.0 125,000 0.0446 2,500 0.33 12,300,000 =OR 0.0020 Alt i,n Alt KA Alt Null Alt Null

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. The ICER of the cable median barrier could also be examined. As before, the null alternative is the unshielded existing median so there is no direct cost associated with alternative j. The ICER for installing a cable median barrier in a 30-foot-wide median on a four-lane highway with 30,000 veh/day is: = + = +    • • ICER DC AP MC OR 125,000 0.0446 2,500 0.0005 =$16,150,000CABLE Alt i,n Alt Alt Null An ICER of $16 million to avoid one KA crash given that the average KA cross-median crash cost (i.e., CKA VSL = 0.33 • 12.3) is $4.1 million would appear to be a poor use of funds. Alterna- tively, if the AADT increases to 60,000 veh/day the number of KA crashes avoided is 0.0051 from Table 49 and the ICER is: = +    • ICER 125,000 0.0446 2,500 0.0051 =$1,583,333CABLE The cable median barrier is much more cost-effective for an AADT of 60,000 (i.e., $1.5 million is much less than $16 million) so it would be a much better use of funds. Notice that these were also the conditions that resulted in a BCR = 2.5. The values in Table 49, therefore, can also be used to calculate the incremental cost-effectiveness ratio. One of the advantages of the ICER method of economic analysis is that it does not require the user agency to monetize fatal and serious injury crashes. Better alternatives have lower ICER values and can be chosen on that basis alone. The ICER can be viewed as a priority rank for various projects, with the higher values representing a high priority and better use of funding. Last, the internal rate of return can also be calculated for comparison for a cable median barrier in a 30-foot width with 30,000 veh/day and the same installation and maintenance costs as above: ∑ ∑ ( ) ( ) + = + + = + → = • • • • • • DC MC PA VSL C OR 1 IRR 125,000 0.0446 2,500 12,300,000 0.33 0.0005 1 IRR IRR =0.2510 Alt Alt i,n KA Alt Null k 0 k k=0 30 k n A traffic volume of 60,000 veh/day results in a value of ORAlt/Null of 0.0051 resulting in the following: ∑ ( ) + = + →• • • 125,000 0.0446 2,500 12,300,000 0.33 0.0051 1 IRR IRR =2.560 CABLE k k=0 30 The IRR for an AADT of 60,000 veh/day is 10 times greater so it would be a much better investment of funds. A highway agency or design engineer would not perform all these different types of analyses but would choose the type of analysis best suited to the highway agency.

Bi-Directional AADT (veh/day) Traversable Median Width (ft) 10 15 20 25 30 35 40 45 50 60 70 80 90 100 Cable Median Barrier 25,000 0.0010 0.0007 0.0007 0.0003 30,000 0.0017 0.0014 0.0013 0.0009 0.0005 35,000 0.0024 0.0020 0.0019 0.0015 0.0010 0.0003 40,000 0.0034 0.0029 0.0028 0.0022 0.0017 0.0009 0.0004 45,000 0.0043 0.0038 0.0035 0.0030 0.0023 0.0015 0.0009 0.0002 50,000 0.0055 0.0049 0.0046 0.0039 0.0032 0.0022 0.0016 0.0008 0.0003 55,000 0.0067 0.0060 0.0056 0.0048 0.0040 0.0030 0.0023 0.0014 0.0008 60,000 0.0082 0.0074 0.0069 0.0060 0.0051 0.0039 0.0031 0.0021 0.0014 0.0003 65,000 0.0096 0.0087 0.0081 0.0072 0.0062 0.0048 0.0039 0.0028 0.0021 0.0009 0.0001 70,000 0.0113 0.0103 0.0097 0.0086 0.0074 0.0060 0.0050 0.0037 0.0028 0.0014 0.0006 0.0000 75,000 0.0130 0.0118 0.0111 0.0099 0.0087 0.0071 0.0059 0.0046 0.0036 0.0021 0.0011 0.0005 0.0002 0.0001 80,000 0.0148 0.0135 0.0127 0.0114 0.0100 0.0082 0.0070 0.0055 0.0045 0.0027 0.0016 0.0009 0.0006 0.0004 85,000 0.0158 0.0144 0.0135 0.0121 0.0107 0.0089 0.0076 0.0060 0.0049 0.0031 0.0019 0.0011 0.0008 0.0006 90,000 0.0158 0.0144 0.0135 0.0121 0.0107 0.0089 0.0076 0.0060 0.0049 0.0031 0.0019 0.0011 0.0008 0.0006 ≥95,000 0.0212 0.0195 0.0183 0.0165 0.0147 0.0124 0.0108 0.0089 0.0075 0.0051 0.0035 0.0025 0.0019 0.0016 Metal Beam Median Barrier 50,000 0.0009 55,000 0.0033 0.0023 0.0022 0.0008 60,000 0.0062 0.0049 0.0047 0.0031 0.0015 65,000 0.0092 0.0077 0.0073 0.0055 0.0038 0.0011 70,000 0.0128 0.0110 0.0104 0.0083 0.0063 0.0034 0.0016 75,000 0.0164 0.0144 0.0135 0.0113 0.0090 0.0057 0.0037 0.0011 80,000 0.0183 0.0162 0.0152 0.0128 0.0104 0.0070 0.0049 0.0021 0.0003 85,000 0.0183 0.0162 0.0152 0.0128 0.0104 0.0070 0.0049 0.0021 0.0003 ≥90,000 0.0292 0.0263 0.0247 0.0216 0.0184 0.0141 0.0113 0.0078 0.0054 0.0033 0.0015 Concrete Median Barrier 80,000 0.0064 85,000 0.0064 90,000 0.0064 ≥95,000 0.0270 0.0162 0.0135 0.0127 0.0094 0.0061 Table 49. Annual fatal and serious injury cross-median crash outcome reduction (KA CMC/mile/year).

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). Table 5-2 in the fourth edition of the RDG presents a list of non-traversable terrain and fixed roadside features that may be considered for shielding with guardrails (AASHTO 2011b). Utility poles, traffic signal supports, trees, non-breakaway signs, and luminaire supports are included in the list. While some roadside features cannot be moved because of size or function (e.g., culverts, canals, and water bodies) others can. Features like utility poles can be moved to a less risk prone location, and features like trees or isolated boulders can be removed. While the “relocate” strategy is clear in principle, in practice it can be difficult to implement because the engineer does not know how much farther from the road the obstacle needs to be moved to make an appreciable difference in safety. In addition, the highway agency is limited by the right-of-way available at each location. Substituting the appropriate values into Equation 2 (see Appendix B.2) results in the following estimate for the frequency of fatal and serious collisions on a 1,000-foot-long segment of roadway: ( ) ( ) ( )( ) =               + −        • • • • OUTCOME BEF EAF 5,280 PSL 65 0.0589 P W 1,000 0.3508 P W P W FO S S s 3 3 y F FO y F FO y BFO The expected annual OUTCOMEFO values for a posted speed limit of 65 mph and flat approach terrain for 1,000 feet of divided and undivided roadways are shown in Table 50. Table 50 can be modified for other posted speed limits by multiplying by (PSL3/653), and all the encroachment adjustment factors can be multiplied directly with the shown values to account for site condi- tions. Moving a fixed object farther away from the traveled way is always a safety benefit, but since the fixed object is small (i.e., Table 50 assumes a one-foot diameter), the annual expected number of fatal and serious collisions is likewise small. Table 50 indicates that the expected annual number of KA ROR crashes with a single pole (i.e., fixed object) located 10 feet from the edge of travel on a 1,000-foot segment of a two-lane rural road is 0.00009 KA ROR crashes/1,000 ft/year. If there were six such poles located on the 1,000-foot segment (i.e., an aver- age pole spacing of 200 feet), 6 • 0.00009 = 0.0005 KA ROR crashes/year would be expected. If the poles were moved back 6 more feet to an offset of 16 feet, 6 • 0.00008 = 0.00048 KA ROR crashes/ year on this 1,000-foot segment. Moving the poles six feet farther from the roadway results in a relative risk of 0.00048/0.0005 = 0.96 or a relative risk reduction of (0.00050 − 0.00048)/0.00050 = 0.04 or 4%. While moving the poles six feet farther from the road reduces risk, it reduces it very little in this case. An economic analysis can also be performed similar to those shown in the last section. Moving a single utility pole can cost between $2,000 and $200,000 depending on the circumstances involved (Baye 2012). Assuming moving a pole costs $50,000, there is no annual maintenance required, the design life is 30 years, and the rate of return is 2%, the following economic measures can be calculated: ( ) = +       = −     = • • • • • • ò BCR OR C VSL DC AP MC 0.00050 0.00048 0.33 12,300,000 50,000 0.0446 0.04 1 RELOCAte Alt Null KA Alt i,n Alt As shown by this example, sometimes there is little benefit to relocating isolated narrow fixed objects because the probability of striking them is already very low.

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. 1987). Sloped object-free terrain shielding guidelines were examined using the method outlined in Chapter 3: Roadside Risk Design Methodology. The relative risk of shielding the object-free terrain with a longitudinal barrier with respect to leaving the terrain unshielded is found as follows: IF 1 P W P P W P 1 THR THR P W P 1 THR 7y BAR SEVBAR y TER SEV TER TER BAR y TER SEV TER TER ( )( ) ( ) ( ) ( ) ( ) ( ) > + −  −  • • • • • • THEN Install a longitudinal barrier to shield the object-free sloped terrain. ELSE Leave the object-free sloped terrain unshielded. Offset to Face (WF) (ft) Annual Expected Number of KA Crashes Offset to Face (WF) (ft) Annual Expected Number of KA Crashes Offset to Face (WF) (ft) Annual Expected Number of KA Crashes 2-Lane Undivided 4-Lane Divided 2-Lane Undivided 4-Lane Divided 2-Lane Undivided 4-Lane Divided 1 0.00010 0.00017 16 0.00008 0.00013 31 0.00006 0.00009 2 0.00010 0.00017 17 0.00007 0.00012 32 0.00006 0.00009 3 0.00010 0.00016 18 0.00007 0.00012 33 0.00005 0.00009 4 0.00010 0.00016 19 0.00007 0.00012 34 0.00005 0.00009 5 0.00009 0.00016 20 0.00007 0.00012 35 0.00005 0.00009 6 0.00009 0.00015 21 0.00007 0.00011 36 0.00005 0.00008 7 0.00009 0.00015 22 0.00007 0.00011 37 0.00005 0.00008 8 0.00009 0.00015 23 0.00007 0.00011 38 0.00005 0.00008 9 0.00009 0.00014 24 0.00006 0.00011 39 0.00005 0.00008 10 0.00009 0.00014 25 0.00006 0.00011 40 0.00005 0.00008 11 0.00008 0.00014 26 0.00006 0.00010 41 0.00005 0.00008 12 0.00008 0.00014 27 0.00006 0.00010 42 0.00005 0.00007 13 0.00008 0.00013 28 0.00006 0.00010 43 0.00004 0.00007 14 0.00008 0.00013 29 0.00006 0.00010 44 0.00004 0.00007 15 0.00008 0.00013 30 0.00006 0.00010 45 0.00004 0.00007 0.00000 0.00005 0.00010 0.00015 0.00020 0 10 20 30 40 50 A nn ua l E xp ec te d N um be r o f K A C ra sh es Lateral Offset to Face of Narrow Fixed Object (ft) 2-Lane Undivided Highway 4-Lane Divided Highway Table 50. Expected annual number of fatal and serious injury crashes with a narrow fixed object 1-foot diameter or less on a 1,000-foot highway segment with undivided roads with AADT > 5,000 veh/day and divided roads with AADT > 24,000 veh/day and posted speed limits of 65 mph.

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 sub- scripts indicating the following: BAR = Shielding longitudinal barrier. TER = The roadside terrain. Full details are in NCHRP Research Report 996 and the derivation and full equations are shown in Appendix B.3 (Carrigan and Ray 2022). A general procedure for determining whether shielding the object-free terrain with a longitudinal barrier is beneficial can be determined using Equation 7 based on the following assumptions: • The shielding barrier is a metal-beam barrier (i.e., PSEV BAR = 0.0091, δBAR = 0). • The proportion of heavy vehicles in the traffic mix is less than 5% so the shielding barrier has a negligible probability of penetration by passenger vehicles (i.e., THRBAR = 0). • The foreslope begins at the back of the shielding barrier and ends at the face of the fixed object or the next change in slope. • The crash severity of a rollover on the terrain from Table 45 and the probability of passing all the way through the sloped terrain (THRTER) from Table 43 (i.e., PSEV TER = 0.0589, δTER = 1). • The posted speed limit is 65 mph. When these typical values are substituted into Equation 7 the relative risk of the shielding guardrail with respect to the unshielded sloped terrain was always greater than one, usually much greater. This indicates that for foreslopes between −12:1 and −2:1 and offsets to the bottom of the slope up to 100 feet wide, shielding with a guardrail is likely to do more harm than good if the slope is free of fixed objects and there are no fixed objects at the bottom of the slope. This analytically confirms earlier research that concluded shielding slopes that are otherwise free of fixed objects with a longitudinal barrier does not reduce the risk of a fatal or serious injury crash, even for foreslopes as steep as −2:1. There is no question that a rollover is a more severe crash than a crash with a w-beam guardrail. A crash with a guardrail, however, is much more prob- able than a rollover on an unprotected slope due to the proximity of a guardrail to the roadway edge and the independent probability of rollover if a vehicle interacts with the sloped terrain. This research found that longitudinal barriers should not be used to shield foreslopes flatter than −2:1 if the foreslope is smooth and otherwise free of fixed objects and there are no obstacles at the bottom of the slope. 4.4 Shielding Fixed Objects with Longitudinal Barriers Fixed object shielding guidelines were also developed using the method outlined in Chap- ter 3: Roadside Risk Design Methodology, by Carrigan and Ray. The relative risk of shielding the fixed object with a longitudinal barrier to leaving the fixed object unshielded is found as follows: IF >1 OUTCOME OUTCOME SHLD UNSHLD (See full equation in Appendix B.3.) 8 THEN Install a longitudinal barrier to shield the fixed object. ELSE Leave the fixed object unshielded. where terms are as previously defined for Equation 2 in Section 3.3.1: Method, with the sub- scripts indicating the following: SHLD = The alternative with a shielding longitudinal barrier. UNSHLD = The unshielded fixed object (i.e., the Null alternative). BAR = Shielding longitudinal barrier. TER = The roadside terrain. FO = Fixed roadside object.

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). Table 5-2 in the fourth edition of the RDG presents a list of non-traversable terrain and fixed roadside features that may be considered for shielding with guardrails (AASHTO 2011b). The guidance is general and does not include information on the size or placement of the features. The list is diverse in that it includes narrow fixed objects like utility poles, trees, and luminaires; continuous features like retaining walls and parallel drainage ditches; and bodies of water and foreslopes. Often, the guidance in the RDG simply resorts to a recommendation for the design engineer to use engineering judgment based on the “size, shape and location of [the] obstacle” (AASHTO 2011b). The method proposed earlier in Chapter 3: Roadside Risk Design Meth- odology, was used to develop more quantitative recommendations that are explicitly dependent on the “size, shape and location of [the] obstacle” (AASHTO 2011b). Like the guidelines for medians presented in Section 4.1: Median Barrier Selection, a relative risk approach was adopted, which compared the risk of the shielding barrier to the risk of not shielding the roadside feature. A shielding barrier should only be used when the shielding alter- native results in less risk of a fatal or serious crash than leaving the roadside feature unshielded. For example, a non-breakaway traffic signal support may be located close to the roadway and present a significant risk to motorists. Shielding the traffic signal support should only be con- sidered when the shielding barrier results in less harm than the unshielded support. Installing hundreds of feet of shielding barriers may, in some situations, actually result in increasing rather than decreasing the risk of a fatal or serious injury crash. In short, the ratio defined above should be less than one to ensure the shielding barrier does more good than harm. A general procedure for determining whether a longitudinal barrier is beneficial can be deter- mined using Equation 8 based on the following assumptions: • The shielding barrier is a metal-beam barrier (i.e., PSEV BAR = 0.0091, δBAR = 0). • The shielding barrier has a negligible probability of penetration by passenger vehicles (i.e., THRBAR = 0). • The shielding barrier is located 4 feet from the edge of travel (i.e., WF BAR = 4 ft). • The shielding barrier is intended to intercept 95% of encroachment trajectories. • The shielding barrier includes approach barrier on the upstream end with a terminal that extends 12.5 feet upstream of the end of the length of need. • The terminal is a NCHRP Report 350 or MASH tangent (i.e., PSEV TRM = 0.0500, δTRM = 0). • The foreslope begins at the back of the shielding barrier and ends at the face of the fixed object (i.e., PSEV TER = 0.0589, δTER = 1). • All roadside obstacles are assumed to have a crash severity of 0.0589 as discussed in Sec- tion 3.3.2.3.1: Conditional Probability of a Fatal or Serious Injury Crash (PSEVj) (i.e., PSEV FO = 0.0589, δFO = 0). • The posted speed limit is 65 mph. • The 15th percentile encroachment angle (θ15) is five degrees and the 85th percentile encroach- ment angle (θ85) is 22 degrees. Results for traversable and non-traversable slopes for two different relative risk ratios are plotted below in Figure 40. A relative risk of 1.0 indicates that the risk of a fatal or serious injury crash is the same for the shielding barrier alternative as for the unshielded roadside feature. A relative risk of 0.75 indicates that the shielding guardrail results in a 25% risk reduction with respect to the unshielded roadside feature. While the shielding barrier may be risk beneficial (i.e., 1.0), it may or may not be cost-beneficial as will be discussed shortly. A site with an isolated individual narrow fixed object like a utility pole is depicted in Fig- ure 41. The variables θ15 and θ85 represent the 15th and 85th percentile encroachment angles.

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”). These values are similar though a little smaller than those found in a number of other older studies as reported in NCHRP Report 665 (Mak et al. 2010). For example, the 50th percentile (i.e., mean) encroachment angle in the NCHRP Report 655 data was just under 17 degrees, whereas in Mak’s study of utility poles it was just under 15.9 degrees. And as mentioned above, the NCHRP Project 17-43 data puts it at 11 degrees. The 15th percentile is used at the approach end to maximize the length of trajectories that may strike the fixed object, whereas the 85th percentile is used at the trailing end of the fixed object to maxi- mize the effective length of the object. The choice of which percentile to use is a design decision that can be changed although the 15th and 85th percentiles are a reasonable place to start. As an example, an engineer may consider shielding a single isolated tree. This situation is represented by the layout in Figure 41. Assuming the tree is about 1 foot in diameter, the value for the length of fixed object (LFO) is about 1 foot and is represented by the solid lines in Fig- ure 40. Since the tree is roughly circular in cross-section, the width perpendicular to the roadway (i.e., WB FO – WF FO) is also about 1 foot which corresponds to a line just above the x-axis in Fig- ure 40. The top two graphs in Figure 40 represent the condition where a shielding metal-beam guardrail is associated with the same risk as to the unshielded fixed object. If the tree is 10 feet from the road, the shielding guardrail provides no benefit and is no better than leaving the tree unshielded. For isolated single trees closer than 10 feet from the road there is some risk reduction from installing a metal-beam guardrail, but the engineer would like to know how much. The bottom row of Figure 40 shows graphs for traversable (i.e., lower left) and non-traversable (i.e., lower right) terrain for a relative risk of 0.75. For points that plot to the left of a line in the lower graphs in Figure 40, the shielded alternative has at least a 25% reduction in risk over the unshielded alternative. As shown on the bottom left graph in Figure 40, shielding a 1-foot-diameter tree with a metal beam guardrail will never attain a 0.75 relative risk. In other words, installing several hundred feet of guardrail to shield a small-diameter isolated hazard is seldom risk beneficial. This was also shown earlier in Section 4.2: Relocating Narrow Fixed Objects, where it was shown that the expected number of KA crashes for a narrow fixed object like a tree was small. Figure 40 and Figure 41 can also be used for larger hazards like a roadway passing near a water body. Assume a designer is considering the need for a metal-beam guardrail to shield a small pond. The edge of the pond is 20 feet from the edge of the roadway and extends parallel to the roadway for 100 feet and is more than 10 feet wide. The roadside between the roadway and the pond is traversable and the designer wants to achieve at least a relative risk of 0.75 (i.e., a 25% reduction) so the graph at the lower left of Figure 40 is used. The lateral offset of 20 feet and width of greater than 10 feet plots to the left of the dashed line representing a 100-foot-long feature, so a shielding guardrail is expected to reduce the risk of a KA crash. Interestingly, the 0.75 relative risk line for long features (i.e., 100 feet and greater) is a vertical line at a lateral offset of 32 feet. This is essentially identical to the conventional 30-foot clear zone suggested by the 1967 AASHO Yellow Book (AASHTO 1967). Figure 40 and Figure 41 can be used to assess the effectiveness of metal-beam barrier shielding for hazards of any size and orientation. Graphs similar to Fig- ure 40 could be produced for concrete and cable barriers by changing the value of PSEV BAR in the OUTcome_shld and OUTCOME_unshld terms of equation 8. Figure 42 shows another common situation where multiple narrow fixed objects are consid- ered for shielding with a barrier. A line of utility poles or a series of bridge piers are two examples of multiple fixed objects that are represented by Figure 42. The vehicle width is accounted for in the lateral distance to be the back of the feature (WB FO) and the length of the feature (Lj FO) in the

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 ] Lateral Offset to Front of Fixed Object (ft) [WF FO] Traversable Slopes | Relative Risk = 1.0 CO N SI D ER SH IE LD IN G N O T N O RM AL LY S H IE LD ED 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 W id th o f F ix ed O bj ec t ( ft ) [ W B FO - W F FO ] Lateral Offset to Front of Fixed Object (ft) [WF FO] Nontraversable Slopes | Relative Risk = 1.0 CO N SI D ER SH IE LD IN G N O T N O RM AL LY S H IE LD ED 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 ] Lateral Offset to Front of Fixed Object (ft) [WF FO] Traversable Slopes | Relative Risk = 0.75 CO N SI D ER SH IE LD IN G N O T N O RM AL LY S H IE LD ED 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 W id th o f F ix ed O bj ec t ( ft ) [ W B FO - W F FO ] Lateral Offset to Front of Fixed Object (ft) [WF FO] Nontraversable Slopes | Relative Risk = 0.75 CO N SI D ER SH IE LD IN G N O T N O RM AL LY S H IE LD ED Figure 40. Fixed object risk beneficial curves for a relative risk of 1.0 and 0.75 (Carrigan and Ray 2022).

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. If the spacing is smaller, the multiple narrow fixed objects are considered one long object. Figure 43 shows the critical spacing that determines when multiple narrow fixed objects should be treated as individual narrow fixed objects or a composite fixed object with a length equal to the sum of all the spacing between the objects and the length of the objects. Figure 43 is based on the work of Johnson et al. who developed risk corridors that define the length in advance of a feature where shielding would need to be provided to intercept a fixed percentage of the encroachments that would interact with the fixed object (Johnson et al. 2015). The length of guardrail needed to intercept 95% of the trajectories that would interact with the fixed object on roadways with 55 mph or higher posted speed limits was found to be: = θ L W tan 9R BFO 15 A line of multiple narrow fixed objects with spacing less than this distance can be considered one object where the length of the fixed object is the sum of all the spacings and the length of the objects themselves. For example, a line of five utility poles located on average 10 feet from the edge of travel and spaced 200 feet apart would plot above the line in Figure 43 so each pole should be considered an isolated fixed object and, as shown in the previous example, would likely not need shielding. On the other hand, if the spacing of the five utility poles were 150 feet at the same 10-foot offset, the point plots below the line in Figure 43 so the line of poles should be considered one isolated 150 • (5 − 1) = 600-foot-long object when using Figure 40. In this situation, the width of the feature is less than one foot, so it lies on the x-axis of Figure 40. The small, dashed line in Figure 40 represents a feature-length of at least 100 feet, and the width of the features is less than one foot corresponding to the x-axis. According to the lower-left graph in Figure 40, shielding five utility poles offset 10 feet from the road with metal-beam guardrail spaced 150 feet apart should result in more than 25% risk reduction (i.e., a relative risk of less than 0.75) since this point plot well to the left of the small, dashed line. Features like rivers, canals, and walls can be evaluated with Figure 40 as well. The width of the vehicle need not be accounted for in these cases because the vehicle cannot get behind the feature like it can for a fixed object like a bridge pier, utility pole, or tree. For example, the lower- left portion of Figure 40 indicates that a 10-foot-wide canal (WB FO – WF FO = 10 feet) parallel to a roadway for 100 feet along its length with a traversable slope should be considered for shielding Figure 41. Evaluating shielding for an isolated fixed object (Carrigan and Ray 2022).

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) open-channel drainage ditch that crosses a 50-foot-wide traversable [i.e., (WB FO – WF FO = 50 feet)] median should be considered for shielding at the 0.75 relative risk level. The decision of what relative risk should be chosen is a policy decision for highway agencies. A relative risk of unity indicates that the shielding barrier does no more harm than the unshielded feature, but it also does not reduce the risk of a KA crash. A relative risk of 0.75 means that Figure 42. Evaluating shielding for multiple fixed objects (Carrigan and Ray 2022). 0 100 200 300 400 500 600 700 0 10 20 30 40 50 Sp ac in g be tw ee n M ul tip le F ix ed O bj ec ts ( ft) [S FO ] Offset from Edge of Lane to Back of the Fixed Object (ft) [WB FO] Treat Multiple Fixed Objects as One Continuous Object Treat as Isolated Single Fixed Objects Figure 43. Critical spacing of multiple narrow fixed objects (Carrigan and Ray 2022).

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. Charts like Figure 40 could be produced for any relative risk between about 0.7 and 1.0. The relative risk can also be used directly in a benefit-cost approach recognizing that the reduction in KA crashes is equal to: ( )− = − •OUTCOME OUTCOME 1 RR OUTCOMENull Alt Null The approach outlined above, therefore, can be used directly where a highway agency chooses an explicit relative risk goal (e.g., 0.75 or less) or determines the need for shielding based on when the barrier does no more harm than the unshielded object (i.e., relative risk = 1) and then determines whether the shielding barrier is cost-beneficial. In either case, the decision to shield or not to shield a roadside feature is based on the quantified risk of observing a KA crash. The approach outlined above can be used to develop quantitative guidelines for shielding roadside features of a wide variety of roadside features rather than depending on engineering judgment as is currently the case in Table 5-2 of the RDG. 4.5 Bridge Rail Selection Design guidelines for median barrier selection and roadside feature shielding were developed in the previous sections based on the relative risk. The risk of the roadside hardware improve- ment is compared to the risk of the unimproved existing features, and the hardware is installed only if the risk is reduced. In these cases, there is a safety benefit to the unimproved alternative: wide medians are a safety benefit in and of themselves, and unshielded objects far from the road are unlikely to be struck. In some cases, there is little or no safety benefit to the null alterative. Bridge railing selection is one such case since there is no safety benefit to not having some type of bridge railing to reduce the risk of vehicles leaving the bridge. Any type of bridge railing will reduce the risk of leaving the roadway over not having a bridge railing at all. Ray developed a detailed approach for selecting the most appropriate test level bridge railing to account for the wide variety of site and traffic characteristics that may be encountered (Ray and Carrigan 2014b; Ray et al. 2014d). Ray et al.’s method was the first attempt to develop risk- based guidelines in roadside safety. The method for bridge railing selection is the same as the procedure outlined in Table 8 for general roadway segments. In the case of a bridge, the only roadside feature to evaluate is the potential bridge railing collision and the consequences of penetrating or vaulting over the bridge railing. The selection of the appropriate MASH test level bridge railings for new or rehabilitation construction is dependent on site-specific conditions, and results may differ for each side of the bridge since the curvature and grade adjustments depend on the direction of travel. This pro- cess, therefore, should be followed for each bridge edge. The selection procedures below only apply to the bridge railing itself. Providing appropriate transitions to the bridge rail, adequate shielding of the approaches, and appropriate terminals or crash cushions are also important considerations in the complete safety performance of the bridge. One key component to selecting the appropriate test level is characterizing the area beneath the bridge. A bridge rail penetration has the potential for increased risk based on the land use of the surrounding area. The area beneath a bridge is assigned one of the following three categories based on the land use: HIGH: A high-risk environment includes possible interruption to regional transportation facilities (i.e., high-volume highways, transit and commuter rail, etc.) and/or interaction with a densely populated

Results 159   area. Penetrating the barrier may limit or impose severe limitations on the regional transportation network (i.e., interstates, rail, etc.). Penetrating the barrier also has the possibility of causing multiple fatalities and injuries in addition to the injuries associated with the vehicle occupants. A high-risk envi- ronment is also present if penetration or rolling over the barrier could lead to the vehicle damaging a critical structural component of the bridge (e.g., a through-truss bridge). MEDIUM: A medium-risk environment includes possible interruption to local transportation facil- ities, large water bodies used for the shipment of goods or transportation of people, and/or damage to an urban area which is not densely populated. Penetrating the barrier would limit local transportation routes; however, detours would be possible and reasonable. Penetrating the barrier has the possibility of causing at least one non-motor vehicle injury or fatality. LOW: A low-risk environment includes water bodies not used for transportation, low-volume trans- portation facilities, or areas without buildings or houses in the vicinity of the bridge. Penetrating a barrier in a low-risk environment would have little impact on regional or local transportation facilities. A low-risk environment has no buildings or facilities in the area which present possible non-motor vehicle related victims if a barrier is penetrated (Ray and Carrigan 2014b). A general procedure for selecting the appropriate test level for a bridge railing can be determined using Equation 2 based on the following assumptions: • Find the values for the lateral extents of encroachment PY(WF BR) and PY(WF PEN) in Table 41. WF BR is assumed to be 4 feet and the bridge railing is assumed to be 2 feet wide, making WF PEN = 6 feet. • Find the bridge railing crash severity for all test levels in Table 45 (0.0159). • Harm accrues to all vehicles that interact with the bridge railing or that penetrate the bridge railing (i.e., δBR = δPEN = 0). • The penetration probability (THRBAR) is a function of the percentage of trucks and is in Table 42. The derivation of the following equation is shown in Appendix B.5. EF GOAL P W P P W P THRMOD Y FBR SEVBR Y FPEN SEVBAR BAR( ) ( ) ≤   +  • • • where =          • • • EF BEF EAF L T 5,280 PSL 65MOD S S S S s 3 3 and TS is a factor to account for opposing vehicles crossing the centerline and striking the bridge rail. TS is 1 for divided roadways and 1.7261 for undivided roadways. Several changes were made to Ray’s original procedure. First, he used a performance goal of risk at 0.01 KA bridge rail crashes over the 30-year life of 1,000 feet of bridge edge. Ray had little to base this value on at the time. Below, the absolute risk goal of 0.0325 KA bridge rail edge crashes/mile/year has been used to be consistent with the other recommendations in this report. Converting from the 30-year, 1,000-foot basis to an annual per mile basis indicates that 0.0325 KA crashes/mile/year is roughly equivalent to 0.023 KA crashes/30-year/1,000 feet of bridge edge, about twice what Ray originally recommended. Ray also had difficulty finding crash severity data for bridge railing penetrations. While bridge rail penetrations from several states were collected, these events were very rare. This is still the case, but to coordinate with other recommendations in this report, the severity value assigned to a LOW-risk environment was changed to 0.0589 to match the values for a rollover in Table 45. It has been assumed that penetrating a bridge railing where the risk environment beneath the bridge is characterized as LOW would be similar to a rollover crash. On the other hand, a HIGH-risk environment is one where a fatal or serious injury is nearly assured.

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) using a method appropriate for the highway agency. A design life of 30 years is recommended in the absence of other information. 2. Base Encroachments: Estimate the number of base encroachment frequencies (BEFS) using Table 26 using the two-way design year AADT from Step 1 and the highway type. 3. Site Conditions: Calculate the site adjustment factors by using the appropriate encroachment adjustment factors from Table 27 and inserting the values into Equation 5 to determine the total encroachment adjustment EAFTOT. 4. Modified Encroachments: Calculate the expected modified number of encroachments (EFMOD) by multiplying the BEFS from Step 2 by the EAFTOT from Step 3. 5. Test Level Selection: Characterize the risk environment under the bridge as HIGH, MEDIUM, or LOW according to the definitions provided above. 6. Select Test Level: Select the appropriate test level for the traffic and site conditions by plot- ting the modified number of encroachments (NMOD ENCR) and PT on the appropriate risk-level chart in Figure 44. Enter the appropriate chart in Figure 44 for the risk environment selected above in Step 5, the EFMOD from Step 4, and the PT from Step 1 to select the appropriate MASH test level for the bridge railing. If the point plots above the dashed risk boundary refer to Step 8. If the posted speed limit (PSL) is 45 mph or less and the point plots in the TL3 por- tion of Figure 44, then a TL2 bridge railing may be used. In addition to this risk-based selection criterion, Ray also provided some additional consid- erations as follows— 7. Additional Considerations: The bridge railing selected using this process provides a solution where the expected frequency of a fatal or severity injury crash is less than 0.0325 fatal or severity injury crashes/edge-mile/year when the specific site conditions are evaluated (i.e., traffic volume and mix, geometry, PSL, access density, etc.) are considered. Engineering judg- ment should be used when unusual or difficult to characterize site conditions are encountered when selecting a bridge railing. Limited numbers of crash-tested bridge railings are avail- able at some test levels; therefore, it is possible that the recommended test level barrier for the evaluated site conditions may not be the best choice for some site conditions not explicitly addressed in these selection guidelines. For example, the particular layout of the barrier at the end of a ramp may influence intersection sight distances and require the use of engineering judgment in designing the interchange to determine an appropriate barrier as it approaches the intersection. Another example might be the presence of pedestrians or bicyclists who might benefit from a taller or different type of railing or the use of sidewalks. Some of the factors that should also be considered are: i. TL-5 bridge railings may be appropriate for specially designated hazardous material or truck routes regardless of the results of the above procedure. ii. Intersection sight distance obstructions created by higher test level bridge railings at the ends of ramps or bridges should be considered, and the bridge railings may require tran- sitioning to a lower height approaching the intersection. iii. Stopping sight distance on bridges where the radius and design speed plot below the dashed line in Figure 45 may limit the use of higher test level bridge railings. iv. The presence of pedestrians, bicyclists, snowmobiles, all-terrain vehicles, and other recreational vehicles may affect the choice of the bridge railing. v. Crash history especially as it relates to heavy vehicle crashes or bridge rail penetrations may justify higher performance bridge railings. vi. Regional concerns about snow removal, the hydrological impact of floodwaters flowing over the bridge, and maintaining scenic views may also play a role in the selection of bridge railings beyond these selection guidelines.

Results 161   vii. The capacity of the bridge deck may limit the choices available for higher test level bridge railings on rehabilitation projects. 8. The user may find that the selection plots above the top boundary of Figure 44 (i.e., the clear area). The clear area with no shading indicates conditions for which even a TL-5 bridge railing will not satisfy the goal of 0.0325 KA ROR crashes/edge-mile/year. In such a case the follow- ing questions should be evaluated: i. Can the traffic operational conditions (i.e., AADT and PT) be reduced such that a TL-5 bridge railing would meet the risk goal? ii. Are the roadway characteristics (e.g., horizontal curvature, grade, etc.) resulting in large adjustments to the NENCR? Can the geometry be modified to reduce the adjustments? iii. Can the deck and superstructure support a TL-6 bridge railing? iv. If none of the above alternatives are feasible, a TL-5 bridge railing is likely the most prac- tical alternative available even though it does not meet the desired goal. These situations require a more detailed analysis of the site conditions that examines a broader range of alternatives beyond just the bridge railing test level selection. A solution will probably require the collaboration of traffic operations, geometric design, and bridge railing design engi- neers to either modify the traffic or geometry conditions of the bridge such that these guidelines can be used or perform a crash history investigation to determine the actual performance of the existing bridge railing. As an example, suppose a four-lane undivided roadway crosses a small stream on a 4% down- grade in the primary direction and the highway agency is considering replacing the bridge rail- ing. For traffic volume at 25,000 veh/day with 10% trucks and a posted speed limit of 50 mph, Table 26 puts the expected number of annual base encroachment frequencies at 0.6667. From Table 27, the number of lanes requires an encroachment adjustment factor of 0.91, and the downgrade requires an encroachment adjustment factor of 1.02, so the modified number of encroachments (EFMOD) can be calculated as: EF BEF EAF L T 5,280 PSL 65 0.6667 0.91 1.02 5,280 1.7261 5,280 50 65 0.5MOD S S S S s 3 3 3 3 =         =         = • • • • • • • The point for 10% trucks and a modified encroachment frequency of 0.50 encroachments/ mile/year plots in the MASH TL-3 area of the graph at the left of Figure 44. The modified number of encroachments is also less than the maximum value for a MASH TL-3 bridge railing with 10% trucks in a low-risk environment as shown in Table 51. If the posted speed limit were raised to 75 mph on this same undivided roadway and the trucks in the traffic mix increased to 15%, EF BEF EAF L T 5,280 PSL 65 0.6667 0.91 1.02 5,280 1 5,280 65 65 1.6MOD S S S S s 3 3 3 3 =         =         = • • • • • • • The point for 15% trucks and a modified encroachment frequency of 1.6 encroachments/ mile/year plots in the MASH TL-4 area of the graph at the left of Figure 44. The modified number of encroachments is also less than the maximum value for a MASH TL-4 bridge railing with 10% trucks in a low-risk environment as shown in Table 51. If a bridge railing is on a relatively flat (i.e., grade <|3%|) and straight (i.e., degree of curve < |10|) roadway segment with nominal 12-foot-wide lanes and zero access points/mile (i.e., all EAFS = 1), the abbreviated test level selection table shown in Table 52 can be used. If any of the encroachment adjustments listed in Table 27 are not equal to one, the more detailed approach should be used.

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) M od ifi ed E nc ro ac hm en ts (E nc r/m ile /y ea r) Percent Trucks Medium 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) Percent Trucks High Risk TL3 TL4 TL5 Figure 44. Bridge selection plots for low, medium, and high-risk environments by percentage of trucks (Ray and Carrigan 2014b). Table 51. Maximum modified encroachments per mile per year for each MASH test level bridge railing. Percentage of Trucks (PT) 0 5 10 15 20 25 30 35 40 45 50 Test Levels Low-Risk Environment TL-3 2.31 1.97 1.71 1.51 1.36 1.23 1.12 1.04 0.96 0.90 0.84 TL-4 2.31 2.04 1.83 1.66 1.51 1.39 1.29 1.20 1.12 1.06 1.00 TL-5 2.31 2.31 2.31 2.31 2.31 2.31 2.31 2.31 2.31 2.31 2.31 Medium-Risk Environment TL-3 2.31 1.45 1.05 0.83 0.68 0.58 0.50 0.45 0.40 0.36 0.33 TL-4 2.31 1.60 1.22 0.99 0.83 0.71 0.63 0.56 0.50 0.46 0.42 TL-5 2.30 2.30 2.30 2.30 2.30 2.31 2.31 2.31 2.31 2.31 2.31 High-Risk Environment TL-3 2.31 0.58 0.33 0.23 0.18 0.15 0.12 0.11 0.09 0.08 0.07 TL-4 2.31 0.71 0.42 0.30 0.23 0.19 0.16 0.14 0.12 0.11 0.10 TL-5 2.27 2.27 2.27 2.28 2.28 2.28 2.28 2.28 2.29 2.29 2.29

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). Table 52. Abbreviated bridge railing test level selection for straight, flat alignments on two-lane undivided and four-lane divided roadways. MASH Test Level Risk Environment Two-Lane Undivided Roadways with AADT ≥ 5,000 veh/day Four-Lane Divided Roadways with AADT ≥ 24,000 veh/day Low Medium High Low Medium High 2 or higher PT≤50PSL≤45 PT≤40 PSL≤45 PT≤5 PSL≤45 PT<50 PSL≤45 PT<20 PSL≤45 PT<5 PSL≤45 3 or higher PT≤25 PSL>45 PT≤5 All PSL PT≈0 All PSL PT≈0 PSL>45 PT≈0 All PSL PT≈0 All PSL 4 or higher 25< PT≤35All PSL 5<PT≤10 All PSL PT≤2 All PSL PT≤5 All PSL PT≤2 All PSL PT≤2 All PSL 5 or higher† PT>35All PSL PT ≥10 All PSL PT ≥2 All PSL PT>5 All PSL PT ≥2 All PSL PT ≥2 All PSL Note: PT = Percentage of trucks (%); PSL = Posted speed limit in mph. † TL-5 is also recommended for any route that is designated as a truck or hazardous materials route.