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Thanks for the clarification. I had originally assumed that you were dealing with a situation that resembled "Illustration 2." For the 30-ft wide median with 6H:1V side slopes, it is only necessary to consider median crossovers since the noted median slopes are relatively flat and clear. As such and using Illustration 2, one would place the median barrier system on the top side of either of the 6H:1V slopes. With the barrier system at the top of the slope, vehicles traversing the centerline ditch would not be expected to underride the barrier on the upslope 15-ft away or so from the ditch center. Therefore, we do not believe that it would be appropriate to use a rubrail in combination with the MGS at this location. In addition, rubrails, used in combination with thrie beam transitions, have not been met with a high degree of success when evaluated by large pickup truck impacts.

In a study published in 2012 evaluating vehicle traversals in 4:1 and 6:1 V-ditches, a potential for overriding barriers was observed due to vehicle rebound on the back side of the slope. To date, results of MASH testing of double-sided W-beam have not been published. Double-sided thrie beam has been successfully tested to MASH TL-3 conditions with a median configuration, and some guidelines for installation in conjunction with foreslopes was presented. More details are available at: https://mwrsf.unl.edu/reportResult.php?reportId=381

Similar questions were answered in other Q&A questions, such as question 1949 and 713:

https://mwrsf.unl.edu/q&a/view.php?id=1949

https://mwrsf.unl.edu/q&a/view.php?id=713

The current guidance related to installing MGS or thrie beam adjacent to slopes is provided below:

Multiple studies have been conducted studying the trajectory of vehicles as they traverse through median ditches. From these studies, V-ditches were shown to be the most critical median shape in terms of effecting the height of the vehicle relative the ground, which would affect the ability of the barrier to capture and redirect an errant vehicle. Additionally, three regions of concern were identified within a depressed median. There is a potential for overriding the barrier as the vehicle is airborne and traversing down the front-slope. When the vehicle impacts the back-slope and compresses the suspension, there is a potential for underriding the barrier. Finally, as the vehicle rebounds off the back-slope of the ditch and is subjected to significant roll and pitch displacements, there is a potential for both overriding the barrier and vehicle rollover.

Those critical areas depend on the ditch slope, the width of the ditch, and the relative position of the barrier in the ditch. Below I have placed a trajectory analysis we did for a 24 ft wide 6:1 ditch as an example. It is from the following report - https://mwrsf.unl.edu/researchhub/files/Report19/TRP-03-265-12.pdf. As noted above, override on the from slope would not be an issue, but you can see the bottom out and back slope override areas on the right side of the analysis. Your ditches may be narrower, but those same two potential areas of concern may be present.

We have not determined the minimum distances to the break point at the bottom of the ditch for shallower slopes or a minimum allowable slope at this time. Shallower and lower slope ditches would pose less of a concern, but the safety performance has not been evaluated to date.