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118 Conclusions In the first phase of this research, the analyses tended to confirm that crashes occur more frequently on curves than on tangent sections. However, the limitations of available data did not allow much mining into the effects of the various design features associated with basic curves, much less super- elevated ones, on the propensity for crashes. Given that vehicles are known to leave the road on curves more frequently (e.g., due to loss of side friction, visibility issues, etc.), it is logical to question whether barriers deployed on CSRS on high-speed highways face demands greater than those same barriers do on tangent, normal sections and whether they provide adequate safety. The available sources of crash data do not typically include sufficient details about either the roadway curvature or the nature of barrier type, dimensions, and placement rela- tive to the shoulder for barriers that were impacted. Thus, crash data does not allow barrier safety performance analy- sis for curves. Further, even when state DOTs have roadway geometry and barrier inventories, they may not be able to create the linkages necessary for safety performance analysis. Various sources of data were explored, but none were found to be useful for analyzing the safety of longitudinal barriers installed on CSRS. The review of previous efforts did not reveal specific knowledge or insights about the safety perfor- mance of longitudinal barriers on CSRS. The state DOT survey conducted in the first phase sought relevant information pertaining to the safety performance of longitudinal barriers on curved and superelevated road sections. The information from the state DOTs revealed that specific design standards and practices, and/or in-service performance analyses did not exist. These efforts revealed that most states use the same design, selection, and instal- lation guidance for longitudinal barriers on either tangent or CSRS. Further, the states did not cite concerns that this was inappropriate. There has been an improving understand- ing of vehicle dynamics and its role in crash occurrence and outcomes, so it was recognized that CSRS effects could lead to vehicle-to-barrier interface problems. These facets made it apparent that it would be useful to address the question of whether there were safety issues associated with longitudinal barriers on CSRS. The second phase of the research began with VDA. Sev- eral recent efforts used commercially available VDA tools to effectively analyze the effects of surface conditions on vehicle trajectories when considering vehicle type, suspension, and speed. The VDA simulations focused on vehicles departing the traveled way on CSRS of high-speed highways. While it was recognized that there could be an unlimited number of vehicle road departure paths, these efforts focused on two types of vehicles on roads at speeds of 60 mph. The depar- tures were all considered to leave the roadway toward the high side of the superelevated section, under the assumption that vertical forces would be more likely to cause vaulting or underride if a barrier impact occurred. The curvature and superelevation rates for six categories of curves defined in the Green Book were considered, as were varying shoulder widths and slopes. It was assumed that the barriers would be placed immediately adjacent to the outer edge of the shoul- der. Three common types of longitudinal barriers were analyzed: NJ concrete barriers, G41S W-beam guardrail, and MGS W-beam guardrail. VDA provided information on how the vehicles depart- ing the roadways would interface with the barriers across a broad spectrum of conditions. The results of this analysis were tabulated to show situations where underride or over- ride issues might exist. Full understanding of the interface was not considered necessary, but VDA was useful for high- lighting CSRS conditions where there could be potential barrier safety performance problems. FE simulations were then undertaken to investigate the impact performance (i.e., physics) of the selected vehicles hitting the barriers for the critical CSRS conditions. These simulations focused on the impact conditions and stan- dards for evaluation of barrier performance as prescribed in MASH. The simulations used validated FE models of barriers C H A P T E R 8
119 and vehicles. More than 200 detailed simulations were under- taken to analyze those CSRS that were indicated as poten- tially critical in the VDA efforts. The simulations focused on MASH TL-3 evaluation requirements. A considerable amount of information was generated in the simulations. The outcomes were summarized in a series of tables for each barrier and MASH test condition across the range of conditions simulated. For each CSRS condition (i.e., curve radius and superelevation; and shoulder width and shoulder angle), the results of the simulation runs for impacts with specific types of barriers identified possible situations where barrier safety was an issue. Insights were drawn from these summaries and the follow- ing findings emerged: â¢ After approximately 60 simulations for the NJ concrete bar- rier, the results indicated that most passed MASH Test 3-11 (the most critical test) requirements for CSRS conditions when the barrier was installed normal to road or shoulder. The simulations of impacts with the NJ concrete barrier indicated that it was more prone to fail the crashworthiness requirements for situations where the superelevation was 8% or greater and the shoulder angle was 6% to 8%. â¢ Barriers on less severe CSRS conditions were more likely to meet MASH requirements. This might suggest that current applications of concrete barriers are viable for non-severe CSRS conditions. â¢ Efforts to simulate G4(1S) W-beam barriers for various conditions analyzed showed that the 27Â¾-in.-high barri- ers did not perform as well as other barriers with greater heights. â¢ The simulations of G4(1S) barriers at 27Â¾ in. for CSRS applications showed a propensity for override, as the VDA results also suggested. There were fewer cases of vaulting for the 29-in.-high G4(1S). There were no cases where underride was indicated to be a problem. â¢ Simulations of the MGS barriers (31 in. high) showed no propensity for underride issues with the small car and good performance for limiting override. â¢ Additional simulations for F-shape concrete barriers indicated improved performance over the NJ concrete barriers. The simulation efforts included some additional runs to add depth to the analyses and provide a better understanding of the incremental performance differences between condi- tions. These confirmed the findings reported here but were not documented. The project included a budget for full-scale crash testing to verify the findings of the VDA and simulation efforts. There was considerable discussion about which crash tests to conduct. There were many interesting options, but a very limited testing budget. Ultimately, three tests were conducted. These tests were conducted for the most common type of W-beam barrier and bracketed the pass/fail limits indicated by the simulations. Since the test results were considered similar to those of the simulations, they are believed to confirm that the simulations reflected real-world safety performance of barriers on CSRS. 8.1 Proposed Guidance A considerable amount of information was derived from the VDA, FE simulation analyses, and crash testing. In the end, the challenge was to translate these results into guid- ance for the design, selection, and installation of longitudinal barriers on CSRS. Table 8.1 contains the significant impli- cations and guidance derived for the barriers and CSRS conditions analyzed. Guidance implies an understanding of the implications of vehicle-to-barrier impacts on CSRS. These are included along with the critical elements of guidance (in bold) that evolved from this research. These are subject to further vetting, rewording, and editing consultation with AASHTO committees. It is hoped that this construct offers a useful means to summarize the findings of the multifaceted analyses and those related findings that support the proposed guidance for barrier design, selection, and installation. 8.2 Implications for Current Practice The research did not identify issues with the safety perfor- mance of longitudinal barriers installed on CSRS for situa- tions with larger radii and small shoulder angles. There was evidence that barrier safety performance was more likely to be compromised on short-radius, high superelevation CSRS situ- ations. These are most often found on ramps for interchanges. While it was suggested that more research be undertaken for these situations, state DOTs can provide added degrees of safety by applying the guidelines for design, selection, and installation as summarized in Table 8.1. It is hoped that there will be efforts to incorporate the findings of this research into future versions of the Roadside Design Guide. This will increase the awareness of potential safety issues and allow agencies to make the appropriate improvements in project designs, particularly for special situations, but also for their design standards. The findings are also likely to lead to increased awareness by state DOTs of potential safety issues in design, construc- tion, and maintenance operations. Awareness of the safety issues will enhance the recognition of them in the field and ultimately highlight the need to alter practices to mitigate potential safety problems. For example, state DOTs aware that tight, superelevated curves need special design, operations, and maintenance considerations can incorporate appropriate guidance in their manuals. There will be a need to track safety
120 Aspect Implications and Guidance Elements Barrier Design General â¢ Poor vehicle-to-barrier interface limits the barrier functions in a crash. â¢ Good interface is a necessary, but not a sufficient condition for selection of a barrier type. The degree of increased impact severity needs to be further assessed. â¢ Consider using interface analyses (i.e., VDA) to evaluate special cases or other types of barriers to increase the confidence in the design. â¢ Consider higher barriers to better accommodate larger vehicles for CSRS applications. Concrete Barriers â¢ Concrete safety shapes do not have underride problems, but face slopes can induce rollovers. â¢ Use higher concrete barriers where there is a concern about overrides associated with CSRS features. â¢ Concrete barriers with an appropriate face slope may be considered the most universally effective design for CSRS conditions. â¢ Design concrete barriers with minimum face slope to limit vehicle ride-up and maintain a viable interface area overlap. W-Beam Barrier â¢ The need for a higher barrier is apparent, but increasing the rail height necessitated review of underride potential. â¢ Increases in barrier height are most important for tight curves where excessive speeds are likely to occur (e.g., off-ramps, downhill). â¢ Follow the FHWA Technical Memorandum (dated May 5, 2010) that recommends the nominal height for new installations of G4(1S) barriers be 29 in. for CSRS (Nicol 2010). â¢ Consider 31-in.-high W-beam barrier designs for CSRS situations. Selection Curvature and Super- elevation â¢ Conduct deeper analysis of short-radius, high superelevation CSRS situations. â¢ Limit the use of tight curves with high superelevations. â¢ Consider using higher barriers on CSRS with appropriate underride protection. Shoulder Width and Angle â¢ Limit major changes in shoulder slope to avoid impacting the barrier when the suspension effects can maximize the potential interface area. â¢ Use wider shoulders where slope changes must be large to allow the suspension to stabilize the vehicle before impact. â¢ Limit shoulder angle to comply with AASHTO recommendations to ensure the melting snow flows away from the road. Roadside Slope â¢ Limit the variation of slope change on the roadside for situations where the barrier is not placed adjacent to the shoulder to provide an acceptable interface. Barrier Type â¢ Consider higher (e.g., 31-in.) W-beam barrier designs for CSRS situations. â¢ Select barriers with increased height for tight curves where high speeds are likely to occur. â¢ Consider using concrete barriers with minimum face slope (e.g., F-shape) to reduce risk of rollover. Installation Orientation â¢ Promote use of barrier orientation perpendicular to the roadway for concrete barriers. Placement â¢ Limit the placement of barriers to only the edge of shoulder on CSRS, particularly where there is a slope change going to the side slope. â¢ Use wider shoulders with lower shoulder angles relative to the road on CSRS with short radii and high superelevation. Maintenance â¢ Analysis of the effectiveness of damaged barriers on CSRS is needed. â¢ Further analysis of the relative priorities for barrier maintenance on CSRS may be needed. Table 8.1. CSRS implications and guidance derived based on the results.
121 performance along with changes in highway design, vehicle fleet characteristics, driver behavior, and other factors known to influence safety. 8.3 Needs for Future Research This research successfully analyzed many questions related to the performance of longitudinal barriers on CSRS. The effort effectively demonstrated the usefulness of the VDA to understand the potential vehicle-to-barrier interfaces that are critical to safety performance for a range of CSRS conditions. The VDA efforts analyzed the effects of vehicle type, surface profile changes, and speeds across the range of curve, shoulder features, and roadside slopes relative to interface effectiveness. These provided a basis for barrier height and placement guide- lines. These efforts also highlighted the design conditions, barrier types, and placement options that were likely to be problematic, as a focus for the more time-consuming, physics- based FE analysis of the impacts between vehicles and barriers on CSRS. These simulations demonstrated that the variations in design, barrier, and placement could be accurately analyzed using simulation tools. Over 250 simulations were undertaken to determine those conditions where safety performance might not be adequate. These results were successfully verified by crash tests involving impacts for typical CSRS conditions. The vehicle dynamics analyses and crash simulations results provided a sound basis for new guidance for the design, selec- tion, and placement of longitudinal barriers on CSRS. While the research efforts answered many questions, some remain and others become apparent. Some questions that may warrant future research are listed below by importance: Strong Need â¢ Assess design issues associated with the short-radii (tight) curves and high superelevation cases subject to vehicle trav- eling too fast (e.g., tight ramps prone to over-speed vehicles). â¢ Analyze the implications for SUTs impacting longitudinal barriers on CSRS (e.g., TL-4). â¢ Analyze the implications for tractor-trailer impacts on CSRS (i.e., TL-5 and TL-6). â¢ Consider CSRS barrier performance for short wheelbase SUVs (or vehicles known to be prone to rollovers). Addi- tional simulations or testing for other vehicle types (e.g., mid-sized sedans, very small cars). â¢ Sensitivity analysis to determine if barriers on CSRS warrant special damage severity and repair priorities (e.g., when rails separate from posts, more slack is introduced into the system, which may reduce the ability to hold or redirect vehicles). Important â¢ Determine effects of impacts of barriers on inner or down- side of CSRS. Are there differences in effects for impacts with barriers on the outer or inner sides of the curve? â¢ Assess the effects of CSRS placed on vertical grades. â¢ Conduct detailed analysis of vehicle orientation traversing road departure path on CSRS. â¢ Develop protocol for special MASH testing requirements for CSRS. Other â¢ Determine the sensitivity of barrier performance for other impact speeds and angles. â¢ Determine length of need requirements for longitudinal barriers on CSRS. â¢ Determine implications of reduced side friction on CSRS barrier impacts. â¢ Are there differences in performance where the barrier is adjacent to transition sections? â¢ Determine barrier performance for variations in block- outs, rub-rails, and so forth. â¢ Conduct specific vehicle dynamics and impact force analyses not presented to compare impacts on level terrain with those on CSRS. Such research efforts would need to reflect the ongoing changes in road design, traffic characteristics, vehicle fleets, driver behavior, and federal and state policies and practices.