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As part of work with Maryland State Highway Administration (SHA), we recently developed some modifications to their Standard 605.31 regarding placement details for W-beam guardrail. We reviewed the installation guidelines for curb/guardrail combinations and incorporated the recommendations in the attached. We recently received some comments through their industry review group regarding the term “level terrain” for the area between the back of the curb and the offset guardrail face. The term level terrain was pulled directly from NCHRP Report 1089, in the Summary and Conclusions section on page 217, which states, “it is recommended to maintain level terrain conditions behind the curb to ensure proper safety performance.” The reviewers are looking for us to put a number on that, like 2% or flatter or something similar. However, I don’t see any recommendations on that within the report and I was wondering if you could offer some additional guidance regarding that. I think the concern is making sure that positive drainage is maintained, especially if there’s a sidewalk or path between the curb and the guardrail.

Please let me know if there’s any additional information that you could provide or suggest that I could discuss with SHA.

The topic of terrain slopes between curbs and guardrail has been discussed quite a bit since that report was published.  First off, if there is a positive slope (i.e., upward slope going from curb to guardrail such that water would flow towards the roadway), there are no concerns.  That has been clarified in the TRB paper that was written on the topic, but that has not been published yet.

For negative slopes, we recommend keeping both the slope and the guardrail offsets to a minimum.  The greater the slope and the greater the offset, the greater the risk for vehicle override.  We can’t provide specific recommendations without further research, as sloped terrain was not within the scope of the study.