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112 Development of Guidance for Improved Longitudinal Barrier Design, Selection, and Installation on CSRS 7.1 Background This research investigated the safety performance of barriers installed on CSRS with the intent of developing improved guidance for their design, selection, and installation. The literature review, state DOT survey, and analysis of crash data found little specific guidance for the use of longitudinal barriers on CSRS. In this research, a comprehensive analysis of vehicle trajectories was undertaken to determine the influ- ence of a broad spectrum of superelevation, shoulder slope, shoulder width, and barrier placement on interface effectiveness (see Chapter 4). A large number of detailed crash simulations were subsequently undertaken to determine whether possible interface problems could adversely affect crashworthiness (see Chapter 5). The validity of these simulations was confirmed by the results of full-scale crash tests (see Chapter 6). These efforts have produced a wealth of results. Translating the findings into meaningful guidance is the final task of this research. It is necessary to start by carefully defining the meanings of the following three guidance aspects that were included in the research objectives: â¢ Design. The basic design of a barrier must accommodate the nature of the impacts that might be expected. The fea- tures of the design include materials, dimensions, shape, connections, and anchorage. These need to be appropriate for the type of service (i.e., crashworthiness) the hardware is expected to provide. â¢ Selection. Decisions on which barrier to use in a given situation is a function of factors ranging from the road geometry, superelevation, grade, shoulder configuration, and roadside environment. Selection must also consider the nature and speed of traffic, environmental conditions, and crashworthiness requirements (i.e., test level). â¢ Installation. A properly designed and selected device must be appropriately installed and maintained to meet its per- formance expectations. Barriers must be properly placed on the roadside, given the intended orientation, and effectively connected to other barrier elements. Barriers may need maintenance and repairs after critically damaging impacts. Frequent impacts may indicate that there are other condi- tions that need attention or that an upgrade is warranted. The ultimate safety performance of a barrier is a function of the appropriate combination of these three aspects. These are important for developing construction plans for new facilities, as well as assessing the appropriateness of deployed barriers. The following sections describe trends or patterns from the results that can be translated into guidance for each of these aspects. The focus was on crashworthiness; however, the guidance will need to be adapted to practices and policies in each state. CSRS are routinely used for the design of all types of curved road of all classifications. Superelevation (i.e., curve banking) is used to make it easier or more comfortable for drivers to navi- gate their vehicles through curves at higher speeds. While the Green Book provides criteria for the selection of superelevation rates as a function of curvature radius and highway speeds, there is no guidance provided relative to treatments for the adjacent roadsides. The Roadside Design Guide provides only general guidance for the design, selection, and placement of longitudinal barriers and does not offer specific barrier guid- ance for CSRS. It is appropriate from a comprehensive safety perspective to ask whether curve and superelevation design influence barrier performance, and if so, why and by how much. 7.2 Research Questions Many questions can be posed about safety performance of longitudinal barriers on CSRS. Concerns arise when longi- tudinal barriers are installed on CSRS because of the influ- ence the curve has on the angle of impact of a vehicle with respect to the barrier. For example, the angle can increase or decrease as a result of the impact occurring on the inside or outside of the curve. Increases in impact angle may also increase the potential for vehicle instability resulting from C H A P T E R 7
113 wheels snagging on posts and may reduce the barrierâs ability to contain and redirect the impacting vehicle, thus resulting in vehicle penetration or override of the barrier. Variations in the nature of the impact may increase the impact forces that potentially exceed the load capacity of barriers designed for impacts along tangent roadway sections. These conditions may also adversely influence occupant risk. There may be design concerns about rain and melting snow on the shoulders running onto the traveled way, where it could refreeze. For superelevated sections, then, it is com- mon practice to slope the shoulders so that melting snow will drain away from the pavement. The use of higher rates of superelevation limits the shoulder slope options that would allow drainage away from the roadway surface on the high side of the curve. Additional drainage concerns arise for con- crete barriers that trap water along the base of the barrier if there are no adequate drain ports. Another concern related to barrier orientation is when the barrier is installed in true-vertical orientation. The analyses presented in Section 5.6.3 indicated that, under some circum- stances, this orientation resulted in a greater propensity to override the barrier. Better containment seems to be provided when the barrier is installed with an orientation perpendicular to the road surface. There may be additional orientation com- plications for barriers installed on the lower inside of a super- elevated curve. These concerns were addressed in this research. Table 7.1 contains key questions identified at the outset of the research to be addressed when developing the guidance; Category Question: Safety What are the characteristics of CSRS crashes? How effective are police crash records for identifying CSRS safety problems? What are the statesâ perspectives on this safety issue? What is the relative influence of design, surface, placement, and side slope on safety? Do longitudinal barriers on CSRS have similar safety performance as on straight, flat roads? Are some vehicle types more prone to crashes on CSRS? Are there regional differences in safety performance? Curve Features How does superelevation affect safety for varying radii? How much change (if any) is a function of the curve features (e.g., superelevation, radius, shoulder)? What are the relative effects of curve features? What is the influence of vertical grades on CSRS? Design What design analyses have been undertaken for barriers on CSRS? How much difference is there in safety performance related to height? Interface? To what degree is snagging influenced by placement on CSRS? Is snagging influenced by placement and shoulder hinge on CSRS? What is the difference in impacts between barriers on level terrain and those on CSRS? What is the propensity for higher angle impacts on CSRS? What are the retrofit options that would compensate for interface problems? What is the influence of roadway features (e.g., curvature, grade, cross section, side slopes)? Where barrier performance is found to be inadequate, are there options to improve the barrier to meet requirements? Placement What are the influences of shoulder width and slope? Does true vertical or normal to road surface orientation work best? How much adverse effect is noted for placement perpendicular versus true vertical? What is the effect of curbs? Vehicle How do CSRS features influence the roll, pitch, and yaw behavior of errant vehicles? How does this roll, pitch, and yaw effect translate to changes in the interface point on the barrier? What are the differences for curve features and vehicle types? How does vehicle size and weight affect behavior on CSRS? Trucks? Are there VDA metrics that indicate variations in effects on the vehicle? Evaluation What would be the critical test conditions for assessing barriers on CSRS? How sensitive are the effects for any case? Will NCHRP Report 350 and MASH approved barriers meet crashworthiness requirements on CSRS? Would MASH barriers perform better on CSRS than those in NCHRP Report 350? What is the difference in impact severity between barriers on level terrain and those on CSRS? What is the influence of roadway features (e.g., curvature, grade, cross section, side slopes)? What PIRTs should be considered for crashworthiness evaluation? Table 7.1. CSRS research questions.
114 however, addressing them all was beyond the scope of this effort. Therefore, this research focused on these critical or fundamental questions about longitudinal barrier perfor- mance on CSRS: â¢ What is the effect of curvature and superelevation on longitudinal barrier performance? â¢ What is the effect of shoulder width and slope on barrier performance for various curvature and superelevation conditions? â¢ Are all barriers equally effective in deployments on CSRS? â¢ Is barrier performance influenced by vehicle type? â¢ Does barrier orientation influence its performance on CSRS? 7.3 Summary of Findings A considerable amount of information can be drawn from these results. Key observations from the simulation results are noted below for concrete (rigid) and W-beam (semi-rigid) barriers. 7.3.1 Concrete Barriers â¢ Most failures occurred when barriers were in true-vertical orientation. Simulations showed only one failure when the barrier was oriented normal to the shoulder for the 1100C vehicle and one when the barrier was normal to road for the 2270P vehicle. â¢ More failures were noted with higher superelevation: 12% superelevations had more failures than the 8%, and the 6% superelevation had the fewest failures. â¢ There were more failures on roadways with narrower shoulder widths than on those with wider shoulder widths. â¢ Larger shoulder angles (i.e., a greater difference in the angle between the road and shoulder) led to more failures. â¢ Simulations showed the F-shape concrete barrier had a moderately improved performance over the NJ concrete barrier. 7.3.2 W-Beam Barriers â¢ There were more failures on roadways with narrower shoulder widths than on those with wider shoulders. â¢ Larger shoulder angles (i.e., a greater difference in the angle between the road and shoulder) led to more failures. â¢ G4(1S) W-beam barriers at 27Â¾ in. high did not meet MASH requirements for the range of conditions simulated. It was concluded that their performance would not be acceptable for CSRS conditions. â¢ The G4(1S) W-beam barriers at 29 in. high did meet MASH requirements for most cases where there were wider shoulders. â¢ The higher MGS met the MASH requirements for all cases. The 11 simulations covered a valid cross section of CSRS conditions without a failure. 7.4 Translating Findings and Observations into Guidance The research focused on a representative set of features for CSRS for three types of longitudinal barriers, varying shoulder configurations, and different barrier placement and orientation conditions. This allowed analysis of a broad range of interface and impact scenarios to answer the critical questions and provide a basis for the development of guide- lines for the safe and effective deployment of longitudinal barriers on CSRS. In an attempt to translate these results into guidance, sum- mary tables were generated for the three areas where guid- ance was sought: design, selection, and installation. The guidance is derived based on findings from the vehicle-to- barrier interface evaluations as well as the crash simulation analyses. In addition, it was considered useful to cite other areas where guidance may be needed or to highlight future research needs. Table 7.2, Table 7.3, and Table 7.4 address specific subtopics for design, selection, and installation, respectively, with rows in each table. For each subtopic there is a column that cites (in an abbreviated form) the findings and a second column that cites the related implications and guidance. The possible elements of guidance are indicated in bold. The following caveats apply to Table 7.2, Table 7.3, and Table 7.4: â¢ The findings focused on two types of MASH vehicles: 1100C and 2270P. Vehicles of other sizes were not explic- itly analyzed. â¢ Not all possible CSRS conditions were analyzed, but a representative cross section was provided. â¢ Impacts were oriented to the MASH speed and angle requirements for testing longitudinal barriers at TL-3. â¢ Barriers were typically selected after the roadway alignments had been established. â¢ Agencies have more latitude relative to setting shoulder width and slope.
115 Design Guidance Findings by Topic Implications and Guidance Elements General â¢ Interface areas are a function of the type of vehicle, its speed, the nature of the road, shoulder, and roadside slope surfaces conditions. â¢ There are differences in the interface effectiveness for the three types of barriers. â¢ The effective interface area is unique for each barrier type for any curvature, superelevation, shoulder width, and shoulder slope for each vehicle. â¢ Barriers on CSRS with higher superelevations tend to be prone to failures. â¢ Failures occur more frequently for narrower shoulders. â¢ More failures occur with steeper shoulders. â¢ Poor vehicle-to-barrier interface limits the barrier functions in a crash. â¢ Good interface is a necessary but not sufficient condition for selection of a barrier type. Increases in impact severity need to be assessed. â¢ Consider the differences in interface area provided by available barriers and, to the extent possible, select the type that offers a capture area consistent with the expected traffic. â¢ Consider a higher barrier to better accommodate larger vehicles like SUVs and SUTs. Concrete Barriers â¢ NJ concrete barriers higher than 32 in. did not have a direct override issue at TL-3 because the interface area only varies by 2 in. â¢ The possibility of vaulting exists if the face slope of the barrier promotes vehicle ride-up (true vertical more prone to vehicle vaulting and rollover). â¢ F-shape concrete barriers have moderately improved performance over the NJ concrete barriers. â¢ Use concrete safety shape barriers to avoid underride problems. â¢ Use higher concrete barriers where there is a concern about overrides associated with CSRS features (e.g., sharp curves). â¢ Concrete barriers with an appropriate face slope may be considered the most universally effective design. â¢ Design concrete barriers with minimum face slope to limit vehicle ride-up and maintain a viable interface area overlap. G41S W-Beam Barriers â¢ G41S at 27Â¾ in. high shows a propensity to be overridden, but there were no indications of underride issues. Poor interface indicated by VDA was reflected in simulation results. â¢ The 29-in.-high barrier reduces the potential for vaulting compared with those at 27Â¾ in. â¢ The VDA indicated that there could be underride interface issues for the small car. Poor interface effects were not reflected in MGS crash simulation results. â¢ The 31-in.-high W-beam barrier provided the best override protection for most CSRS and shoulder conditions. â¢ â¢ â¢ The need for a higher barrier is apparent, but increasing the rail height necessitates review of the underride potential. Give priority to low barriers on CSRS over similarly low barriers elsewhere. Increased heights are most important for tight curves where excessive speeds are likely to occur (e.g., off-ramps, downhill). Consider 31-in.-high W-beam barrier designs for CSRS situations to further reduce the potential for override. Table 7.2. Design guidance extracted from the simulation results.
116 Selection Guidance Findings by Topic Implications and Guidance Elements Curvature and Superelevation â¢ Longer radius curves with lower superelevations tend to have better interfaces with barriers. â¢ Barriers on CSRS with higher superelevations tend to be prone to failures. â¢ It is more likely that barriers will have a lower safety performance for the higher speed superelevation designs. â¢ Conduct further analysis of short-radius, high superelevation 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¢ VDA indicated that the greater the change in the slope from the traveled way to the shoulder surface, the greater the influence on the vehicleâs suspension and hence on the potential interface with the barrier. â¢ The width of the shoulder influences the time for spring response to limit bounce and undesirable interfaces. â¢ For the 62-mph impacts studied, failures occurred more frequently for 4-ft shoulders than for 8-ft or 12-ft shoulders. â¢ More failures occurred on roadways with steeper shoulders. â¢ Limit the use of major changes in shoulder slope to avoid impacting the barrier when the spring effect maximizes the interface area. â¢ Use wider shoulders where slope changes must be large to allow the suspension to stabilize the vehicle. Roadside Slope â¢ Changes in slope of the shoulder to the roadside surface can influence vehicle dynamics and the interface with barriers that need to be located further from the edge of the shoulder. 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¢ The 29-in.-high barrier greatly reduces the potential for vaulting compared with the 27Â¾-in.- high barrier. â¢ The 31-in.-high W-beam barrier provided override protection for most CSRS and shoulder conditions. â¢ F-shape concrete barriers have a moderately improved performance over the NJ concrete barriers. â¢ Consider 31-in.-high W-beam barrier designs for CSRS situations. â¢ Increase barrier height for tight curves where excessive speeds are likely to occur. â¢ Consider using concrete barriers with minimum face slope (e.g., single slope, over NJ or F-shape) to reduce the risk of rollover. Table 7.3. Selection guidance extracted from the simulation results.
117 Installation Guidance Findings by Topic Implications and Guidance Elements Barrier Orientation â¢ Concrete barriers installed in true-vertical orientation are more prone to vehicle vaultings and rollovers. â¢ Barrier vertical orientation had less effect on W- beam barrier performance. â¢ Promote the use of orientation perpendicular to the roadway surface for concrete barriers. Placement â¢ Barriers further from the edge of the shoulder will be influenced by the change in angle of the shoulder slope to the back slope. â¢ Barriers seem to function better for wider shoulders with the least angle relative to the road. â¢ Limit the placement of the barrier to the edge of shoulder on CSRS, particularly where there is a slope change going to the side slope. â¢ Use wider shoulder with gentler angle relative to the road on CSRS with short radii and high superelevation. Roadside Slope â¢ Changes in slope of the shoulder to the roadside surface can influence vehicle dynamics and the interface with barriers that need to be located further from the edge of the shoulder. 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. Maintenance â¢ Posts knocked out of alignment, settlement, and other damage were not explicitly analyzed, but are likely to influence crashworthiness. â¢ Analysis on 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 7.4. Installation guidance extracted from the simulation results.