Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers (2022)

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57   Conclusions As discussed in the Literature Review, the NTSB has recommended since 1998 that median barrier guidelines should consider heavy vehicles in the traffic mix. The NTSB recommen- dations were one of the catalysts for this research. The recommendations developed in this research provide guidelines that can be easily implemented in the RDG. These recommended MASH median and roadside barrier need, selection, and placement guidelines will be useful to design practitioners and can also be used by the FHWA and AASHTO to satisfy the NTSB recommendations. The approach to developing the median barrier guidelines in this research used the long-held philosophy of the 1967 Yellow Book, which suggested a barrier “should only be used where the result of striking the object or leaving the roadway would be more severe than the consequences of striking the rail.” (AASHO 1967) More particularly, a median barrier should only be installed if it reduces the expected number of KA crashes on the segment from that of a median with no median barrier. One result of this research was a method for assessing the need for a median barrier in situations where roadside barriers are not otherwise needed. In other words, the only hazard to be reduced is minimizing the chance of a vehicle fully crossing the median and striking or being struck by a vehicle in the opposing travel lanes. As shown in Figure 23, these recom- mendations involve the median width, traffic volume, and barrier material. After the need for a median barrier is assessed according to Figure 23, the proper test level is determined using Table 12 based on the PT in the traffic mix. Chapter 3 of the AASHTO RDGuide addresses clear-zone widths and provides guidelines for shielding slopes that are free of fixed objects. Embankment height and foreslope are conven- tionally considered contributory factors to determining the barrier need for shielding slopes. When fixed objects are present, the RDG suggests removing the object or shielding it. (AASHTO 2011)The RDG terrain guidance dates to a 1967 study by Glennon and Tamburri that consid- ered the relative severity of a crash with a barrier to a rollover crash. (Glennon 1967) The RDG acknowledges that this guidance does not account for the different probabilities of a barrier or rollover crash occurring. The Glennon and Tamburri study considered the probability of roll- ing over on the slope to be unity (i.e., all vehicles that interact with the slope will roll over). This reduced the guidance to a simple comparison of the difference in crash severity of impacting the barrier versus rolling over the slope. This research reconsidered the question whether a barrier is needed to shield slopes by modeling the probability of rollover for various slopes, vehicle types, and encroachment conditions. This re-examination found that barriers should not be used to shield slopes that are unobstructed and otherwise free of fixed objects when the slope is 2:1 or flatter. The injury consequences of striking the barrier outweigh the probability of an injury in a rollover on the slope when the slopes are flatter than 2:1. Guidelines for shielding fixed objects on slopes flatter than 2:1 are presented in Figure 25. C H A P T E R 6

58 Selection and Placement Guidelines for Test Level 2 Through Test Level 5 Median Barriers This research developed proposed guidelines for the selection and placement of a broad range of MASH median barriers (i.e., double-faced) and roadside barriers (i.e., single-faced) used within medians or on the roadside for shielding fixed objects. These proposed selection and placement guidelines address variables like the presence or absence of a barrier, the offset to the barrier, the type and test level of the barrier, the highway and median characteristics, and the traffic characteristics including the percentage of heavy vehicles. These guidelines are necessarily complex to address the broad range of these multiple design variables. Agencies that wish to have the greatest flexibility can incorporate the full extent of the guidelines. Agencies with limited geographic or geometric challenges may find that a simplified version with “built-in” assumptions meets their needs. Before this research effort, roadside safety practitioners have never been able to use the spe- cific site and traffic characteristics to target scarce transportation resources on the particular median sections most at-risk of a CMC. The use of these guidelines is expected to both improve the effectiveness of design guidelines in minimizing CMCs while targeting DOT funds and resources to the highway segments where the most benefit can be achieved.