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Curb and Guardrail Replacement

State IL
Description Text

One of our districts is planning to update the guardrail and replace the curb (from asphalt to concrete curb) shown in the images below. They are asking if the curb is acceptable to be installed in front of the post (behind the guardrail face), as the existing condition? Research have shown acceptable performances when the guardrail face is offsetting from the curb (ranging from 0 to 6”, and 4’ to 12’), but I just want to double check if having the curb behind the guardrail face and in front of the post is also acceptable.


Based on the images, steep slopes seem to be present. Our standard indicates when the slope is steeper than 3:1 and the distance from the back of post to the hinge point is less than 24 in, the post shall be steel, and the embedment shall be 76 1/8 in and the minimum top of rail height shall be 31 in; I think this may be the case on this location.


Thanks in advance for any comments you may want to provide,



W-beam Guardrails


Date June 29, 2022
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Attachment ILDOT-1.jpg Attachment ILDOT-2.jpg

We don’t see an issue with mounting the curb behind the face of the rail. This was actually the standard under NCHRP 350 as it promoted improved interlock of the vehicle with the W-beam prior to impacting the curb. The increased height of the MGS has allowed it to work with installation of the rail 6” behind the face of the curb.


In terms of the slopes, your comment below seems to suggest that you are using 9’ long posts (76” embedment plus 32” for the post). This would be in line with what we have previously tested adjacent to the slope break point of a 2:1 slope. If you are at or near the slope break point 9’ posts may be acceptable. If you have a larger offset closer to the recommended 24”, the deeper embedment can lead to rail pocketing as the post will be much stiffer than a typical W6x9 post that is 6” long.


Additionally, no testing has been conducted with the MGS with a curb adjacent to slope. There would be some concern that the combination of the curb with the slope may affect the barrier performance and vehicle capture. Currently, we have noted that installation of the MGS at the slope break point of a 1V:2H fill slope adjacent to curb may not be MASH TL-3 compliant and may require further research to verify its safety performance. We might recommend limiting the curb height as much as possible to reduce these concerns.


We previously published a research report providing guidance on MGS installations adjacent to slopes that may provide further insight.


Finally, a couple of comments on the installations below.


  1. What is the reason for the ¼ post spacing that can be seen at various locations along the rail? These areas may create locally stiff regions in the barrier that promote pocketing. One may want to consider eliminating these if possible.
  2. The lower picture seems to show some type of luminaire support within the working width of the MGS. We have looked at a similar situation for the Illinois Tollway previously. You may want to review the guidance in that report -



Date June 30, 2022
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