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Can you please offer guidance on whether existing MGS testing is sufficient to consider it as remaining crashworthy under MASH TL-3 with the modification shown in the attached detail? The detail shows standard MGS guardrail with a W-beam rail mounted on the field side of the post to serve as a backside rub rail when positioned at the front of a trail. 12” blockouts are shown on the traffic side of the post, with no blockouts shown on the non-traffic field side of the post.

MnDOT has allowed this on a case-by-case basis, as it combines elements of MGS and 31-inch W-Beam Median Barrier (TTI Test Report 9-1002-12-8), both evaluated as MASH compliant. Our ADA group would like to recommend this configuration to mitigate tripping risk wherever guardrail must be placed directly at the front of a pedestrian facility in constrained settings. However, first we would like your comments on whether this modification to the MGS system compromises its crashworthiness.

Thank you!

We do not believe that the proposed configuration below would pose a serious safety issue. As you noted, the system is very analogous to the median version of the MGS which has been evaluated to MASH at TTI. Older testing of double-sided W-beam and thrie beam systems would also support a low level of safety concerns with this configuration. The only caveat is that the system has not been tested to MASH.

The main concern for a dual rail system like the one shown is that the dual rail will limit the release and knockdown of the guardrail along the weak axis during small car impacts. This can lead to increased vehicle/wheel snag and associated increased vehicle decelerations. To alleviate this concern, one would want to make sure that the rear rail element is connected using similar standard guardrail bolts to maintain adequate rail release. As an alternative, we developed a breakaway bolt option for the MGS on weak (S3x5.7) posts that could be used for the rear rail as well. I can provide details on that connection if you do not have it.

Let me know if you have further questions.

Thanks!