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We recently received a question from a designer regarding the location of GDOT 31” guardrail (MGS) relative to our V-gutter (detail below).  In the past, I believe we have placed the guardrail face at the back of the v-gutter (the 4:1 side) and mounted at 31” relative to the roadway surface.  This is similar to our practice of placing guardrail at the back of our standard curb and gutter sections and mounting at 31” relative to the roadway surface.  This would effectively put the face of the rail ~1.5’ from the V-gutter flow line (maybe slightly more given the vertical curve in the gutter detail).  NCHRP 1089 recommends mounting at 31” relative to the top of curb/soil back fill when 1’ to 6’ from the face of curb.  We just wanted to see if y’all concur with mounting at 31” relative to surrounding backfill given this gutter section with face of rail at back of V-gutter.  We are also interested if you think there is a more appropriate lateral offset and mounting height based on the research findings in 1089.

We did note a previous Q&A with a similar question; however, it predates 1089 by quite a bit.

I agree with you.  I would install the MGS at 31” relative to the soil backfill (surrounding terrain).

Is there a better location to install the MGS?  Safety performance wise, I would say the closer to the curb, the better only because it would limit the time from curb impact to rail impact, thereby limiting vertical trajectory and vehicle roll prior to impact with the MGS.  However, offsetting the MGS further from the curb would limit nuisance hits and costs of maintenance/repair to the MGS.  Depending on the amount of nuisance hits you get, it could be more beneficial to offset the MGS a couple feet further (if you have the space) to save on maintenance while still knowing the MGS is crashworthy up a 6-ft offset. 

Thank you for the reply.  I forgot to respond regarding your question below.  The V-gutter is proposed to be located at the outside of a shoulder, so we should not be dealing with too much of an issue with nuisance hits.  We are developing a detail to address the placement of guardrail in relation to our V-gutter specifically, so we will encourage placement further back where possible; however, I think we will generally be pushing the V-gutter as far from the travel lane as possible.

We had a separate instance arise where some guardrail was installed roughly 8” from the toe of a sloped curb (4” tall, 6” wide).  1089 doesn’t specify a mounting height between 0.5’ and 1’.  Do y’all have any guidance in between?  Is it better to lean toward the mounting height that your offset is closer to (i.e., relative to roadway up to about 9” and then relative to soil backfill from 9” up)?

Good question.  As you noted, Report 1089 did not specifically call out a recommended height for MGS between 6” and 12” from the curb.  This was because we did not have a test to lean on and provide concrete justification for the recommendation. I do think that either of the mounting heights would likely PASS MASH TL-3 in this region, based on the simulation efforts within the study. That said, I believe the results from the simulation efforts showed that the MGS mounted at 31” relative to the soil backfill performed better in terms of vehicle stability than the lower MGS configuration. Thus, I would probably lean toward the use of 31” relative to curb/soil backfill in this region, but again, either is likely crashworthy.