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We are considering requests to allow the offset beyond the
required shoulder width on the interstate to be unpaved in front of
guardrail.  Concerns have been expressed that this approach may affect the
performance of the barrier.  Have there been any studies on the effect of
paving or not paving to the face of guardrail?

I did not see information from the MwRSF Q&A site.

Thanks for the email inquiry regarding paving up to the guardrail’s front face.

With regards to your question, we have not conducted a specific study to evaluate the effect of shoulder paving on both post and guardrail system performance. However, we have conducted a limited number of crash tests where either asphalt or concrete surfacing existed in front of posts.

In general, we try to conduct testing on guardrails with the posts placed a minimum distance away from the roadway edge to eliminate any effects that the surfacing may have on post stiffening. When surfacing is required, we install in a realistic location relative to the posts. For example, we have successfully crash tested (NCHRP 350 and MASH) 31-in. tall approach guardrail transitions with concrete surfacing and curbs placed below the rail and up to the front of the posts. We have also successfully tested (NCHRP 350) 31-in. tall MGS with concrete surfacing and curbs placed below the rail and up to the front of the posts. Unfortunately, we have had an unsuccessful MASH MASH 1100C test when the surfacing continued behind the posts without the use of leave-outs. These studies involved strong steel posts. When weak steel posts are used, there are fewer concerns.

Overall, I believe that road surfacing placed in front of the rail would not be problematic for 31-in. tall strong-post W-beam guardrail systems. With the tests performed thus far, I have not observed any particular problems worth noting. However, the only true method for evaluating this feature would be to perform testing with and without surfacing to provide direct comparisons between systems.

Thank you for your quick reply.  Our concern is more that if the surface to the face of the guardrail is not paved, then rutting or erosion of the non-paved surface may cause the vehicle to strike the guardrail in an unexpected manner – perhaps override/underride.  Perhaps this is more of a traffic safety/driver control issue than barrier performance, per se.  Regardless, I appreciate your input.

Thank you for the response!

I had not addressed the soil grading issue. In general, we would assume that the soil shoulder is compacted and graded without excessive rutting or erosion. Although not evaluated in combination with guardrail, excessive rutting or erosion could contribute to increased vehicle instability during the impact event.