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We have an interstate project that INDOT had originally written into the design build contract that the ZOI should be taken into consideration when placing sound wall, sign structures, bridges, etc.  Given that some of these items, for this question the sign structure (a sign bridge), if placed outside of the ZOI would require more MSE wall construction (the roadway is raised), INDOT is being asked if we would consider placing the sign structure within the ZOI to reduce MSE wall costs.  It appears that Midwest is completing the NCHRP 22-34 report, so we were wondering if you could possibly answer some questions for us? 

• For sign structures (bridges or cantilevers) has any consideration been given that these are spot locations and maybe within a tangent sections, allowing them to be within the ZOI is a low risk and can be considered?


• If a spot locations such as a sign structure was placed within the ZOI, the geometry of the roadway should be taken into account, e.g. outside of a curve it may not be ideal to place the sign structure within the ZOI?


• Does the type of wall, F-shape vs single slope vs vertical, change the ZOI width substantially?


• If a spot location such as a sign structure was placed within the ZOI (inches behind the wall), would increasing the wall height from 45 inches to 54 inches gain any extra protection or just be more money spent?

As always if you would  rather talk about this on the phone, I can set up a Teams meeting.  I did not send this into the general question mailbox because it seemed to be judgement calls on a specific project.  Thank you for your time and consideration.  Katherine

I have commented on your questions below.

Let me know if you need to discuss more.

• For sign structures (bridges or cantilevers) has any consideration been given that these are spot locations and maybe within a tangent sections, allowing them to be within the ZOI is a low risk and can be considered?

That is a consideration that states may take into account. The research in 22-34 is mainly to define what the ZOI is for MASH vehicle and impact conditions given various types of barriers. Your question is more of a policy decision. I am not sure it will be addressed specifically in 22-34. Doing so would likely require some form of cost-benefit or risk type analysis that is not currently within the scope of the project.

• If a spot locations such as a sign structure was placed within the ZOI, the geometry of the roadway should be taken into account, e.g. outside of a curve it may not be ideal to place the sign structure within the ZOI?

This would tend to be a higher risk location based on accident data, so that would be a valid consideration. Similar to my first response, this type of decision making is not part of 22-34. However, I would agree that factors such as curves would be valuable to consider in your policy to minimize risk.

• Does the type of wall, F-shape vs single slope vs vertical, change the ZOI width substantially?

The original ZOI study from 1998 identified different ZOIs based on barrier geometry. The difference was significant at times. For 22-34, we are still working on generating the ZOI envelopes. We do have some preliminary data based on shape. While that work is not complete, it appears that there is not a lot of difference between the barrier shapes for TL-3. For example it appears that the ZOI for single slopes for vertical and safety shape barriers fall around 13 inches. For safety shapes, the number increases to 16 inches, but that is largely due to a single test result. It is more contingent on barrier height.

• If a spot location such as a sign structure was placed within the ZOI (inches behind the wall), would increasing the wall height from 45 inches to 54 inches gain any extra protection or just be more money spent?

Increasing the barrier height would greatly reduce the ZOI, especially for passenger vehicles. We have recommended similar guidance in the past on ZOIs for passenger vehicles and heavy trucks and heights needed to reduce ZOI. See below

                https://mwrsf.unl.edu/q&a/view.php?id=1753

                https://mwrsf.unl.edu/q&a/view.php?id=1254

                https://mwrsf.unl.edu/q&a/view.php?id=1801