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Question
State	IN
Description Text	We have a project were a double-faced MGS w-beam guardrail is being placed within a median with a v-ditch section, slopes 10H:1V and 6H:1V. According to the Roadside Design Guide (RDG), if neither slope requires shielding the median barrier should generally be placed on the side with the steeper slope. We have attached a PDF with the RDG figure 6-18 and section 6.6.1.1. For this project we suggested to place the double-face MGS w-beam guardrail along the 10H:1V slope to comply with the Eligibility Letter B-204. Our FHWA division has scheduled a meeting to discuss this placement and their main question is going to be, "Should we do this for all cases or should we consider a different placement (adjacent the 6H:1V) along sharp horizontal curves?". Do you have any thoughts about this question we could add at the meeting? FYI the meeting is not until March 27th. Thank you

Question

State

IN

Description Text

We have a project were a double-faced MGS w-beam guardrail is being placed within a median with a v-ditch section, slopes 10H:1V and 6H:1V. According to the Roadside Design Guide (RDG), if neither slope requires shielding the median barrier should generally be placed on the side with the steeper slope. We have attached a PDF with the RDG figure 6-18 and section 6.6.1.1. For this project we suggested to place the double-face MGS w-beam guardrail along the 10H:1V slope to comply with the Eligibility Letter B-204. Our FHWA division has scheduled a meeting to discuss this placement and their main question is going to be, "Should we do this for all cases or should we consider a different placement (adjacent the 6H:1V) along sharp horizontal curves?". Do you have any thoughts about this question we could add at the meeting? FYI the meeting is not until March 27th. Thank you

System Performance Evaluation

System Types
Road Closure Gates
Thrie Beam Guardrails

Applications

System Features

Date March 8, 2019
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Attachment INDOT Median Guardrail Placement.pdf

Response
Response (active)	Several items come into play when considering the MGS median barrier installed in an asymmetric median with 10:1 and 6:1 slopes. 1. The MGS median barrier is intended for use on 10:1 or flatter terrain on the approach to the rail as you noted. 10:1 slope have generally been allowed and recommended on the approach for roadside barrier systems. No testing or analysis has been conducted when the barrier is installed at the top of a steeper (i.e. 6:1) slope. For a median installation, 10:1 slopes on each side were recommended to provide similar approach trajectories for each direction of travel. 2. We would tend agree with the RDG that the median barrier should be placed to shield the steeper slope condition. This should allow the barrier to shield the vehicle traversal down the more critical slope. The barrier would need to be placed at the top of the steeper slope at or in front of the slope break point. No testing of the MGS median barrier has been conducted at the slope break point of a 6:1 slope. However, previous MASH TL-3 testing of the MGS with standard 6' long posts was successful at the slope break point of a 2:1 slope. Thus, one would expect adequate vehicle containment for oncoming traffic under MASH TL-3 conditions if the MGS median barrier was installed on the slope break point of a 6:1 slope. It should be noted that this has not been tested, but would be expected to perform safely. 3. This does not consider potential impacts from the reverse direction that would traverse up the 6:1 slope prior to impacting the barrier. This type of impact is currently considered during cable median barrier analysis and would likely need to be considered here as well. The main concern would be the vehicle trajectory causing potential override or underride of the barrier. However, analysis and/or testing has not been performed for this impact condition and further study is likely needed to allay concerns that the vehicle engagement with the barrier could change due to traversal up the slope. 4. In terms of horizontal curves, there may be an argument to shield the shallower slope if it resides on the outside of the curve as more impacts might be expected on the outside of the curve. However, the curves are generally localized and curvature may change in adjacent road sections. Thus, one would need to transition from side to side of the median to achieve consistent placement. This would likely be difficult and lead to barrier placement in the median as the barrier transitioned to the other side in locations where its performance would degrade significantly. Thus, we would not likely recommend it at this time. Let me know if that answers your questions or if you would like to discuss this further. Thanks

Response

Response
(active)

Several items come into play when considering the MGS median barrier installed in an asymmetric median with 10:1 and 6:1 slopes.

1. The MGS median barrier is intended for use on 10:1 or flatter terrain on the approach to the rail as you noted. 10:1 slope have generally been allowed and recommended on the approach for roadside barrier systems. No testing or analysis has been conducted when the barrier is installed at the top of a steeper (i.e. 6:1) slope. For a median installation, 10:1 slopes on each side were recommended to provide similar approach trajectories for each direction of travel.

2. We would tend agree with the RDG that the median barrier should be placed to shield the steeper slope condition. This should allow the barrier to shield the vehicle traversal down the more critical slope. The barrier would need to be placed at the top of the steeper slope at or in front of the slope break point. No testing of the MGS median barrier has been conducted at the slope break point of a 6:1 slope. However, previous MASH TL-3 testing of the MGS with standard 6' long posts was successful at the slope break point of a 2:1 slope. Thus, one would expect adequate vehicle containment for oncoming traffic under MASH TL-3 conditions if the MGS median barrier was installed on the slope break point of a 6:1 slope. It should be noted that this has not been tested, but would be expected to perform safely.

3. This does not consider potential impacts from the reverse direction that would traverse up the 6:1 slope prior to impacting the barrier. This type of impact is currently considered during cable median barrier analysis and would likely need to be considered here as well. The main concern would be the vehicle trajectory causing potential override or underride of the barrier. However, analysis and/or testing has not been performed for this impact condition and further study is likely needed to allay concerns that the vehicle engagement with the barrier could change due to traversal up the slope.

4. In terms of horizontal curves, there may be an argument to shield the shallower slope if it resides on the outside of the curve as more impacts might be expected on the outside of the curve. However, the curves are generally localized and curvature may change in adjacent road sections. Thus, one would need to transition from side to side of the median to achieve consistent placement. This would likely be difficult and lead to barrier placement in the median as the barrier transitioned to the other side in locations where its performance would degrade significantly. Thus, we would not likely recommend it at this time.

Let me know if that answers your questions or if you would like to discuss this further.

Thanks

Date March 15, 2019
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Response
Response (active)	Thank you for your quick response. We would just like to clarify #4 in your response with a picture, see attachment (Clarify #4 in the Response). Also could you give your thoughts on the two double face MGS w-beam guardrail placement options shown in attachment (Double Face MGS W-Beam Placement Options)? Thank you

Date March 18, 2019
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Response
Response (active)	Sorry the attachment did not attach. Can I email the attachments directly to the person that will response? Thank you

Date March 18, 2019
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Attachment Clarify #4 in the Response.pdf Attachment Double Face MGS W-Beam Placement Options.pdf

Response
Response (active)	To address your clarification questions. 1. For the scenario shown in Double Face MGS W-Beam Placement Options.pdf. As noted previously, there is potential justification for placing the barrier at the outside of the median curve closet to oncoming traffic to shield the majority of errant vehicles. However, there is also the desire, as noted in the RDG to shield the more critical slope, which we noted to be a good general recommendation. The first option you have shown in this file indicates placement of the barrier on the 10:1 slope side of the median. While this poses little to no issue for oncoming traffic, it may pose a concern for the reverse direction traffic. Reverse direction traffic would tend to exit the steeper slope and become airborne. This may lead to underride or override of the median barrier depending on the width of the median. The trajectories of vehicles traversing off a 6:1 slope onto the 10:1 as shown has not been specifically analyzed, but we do know that the vehicle might be airborne, have the suspension compressed/bottomed out, or be rebounding from the suspension compressing. All of these items would affect vehicle interaction with the guardrail and may degrade vehicle capture in a manner similar to what I noted previously for traversing up the 6:1 slope. The extent of the concerns is difficult to determine without analysis of the ditch geometry and the vehicle trajectory. In summary, there are potential concerns for vehicle interaction with the median barrier for both directions of the slope traversal that cannot be ruled out without further analysis. In addition, we would tend to agree with the more general recommendation in the RDG to shield the steeper slope, with the caveat that there may be an additional consideration when horizontal curves are installed. The second page of that attachment shows an option where you are shielding the curve of the road closest to oncoming traffic where impacts are more likely to occur with individual barriers on the outer edge of each curve. There still may be issues with ditch traversal in this instance as discussed previously, but these impacts are less likely due to the roadway curvature. In this case the vehicle impacts are more likely to occur with the barrier placed close to the travel way, which would not pose that concern. Overlapping or fish-scaling of the installation as shown also eliminates the need for the barrier to traverse the ditch. We believe that this is likely the better option. 2. For the scenario shown in Clarify #4 in the Response. For s single roadway curve as shown, there is a reasonable argument for shielding the outside edge of the curved median for oncoming traffic as this is the most likely impact scenario regardless of the slope at that location (assuming that both slopes are traversable as shown). As you noted, for a barrier placed to shield the 6:1 slope, there is a higher risk in this situation of the impacting vehicles traversing the ditch and the 6:1 slope prior to impacting the barrier due to the curvature. This would cause some of the concerns for vehicle capture noted previously. Thus, placement of the barrier to shield the shallower slope in this instance may provide increased safety benefit due to the road curvature. Let me know if that clarifies things or if you need further discussion. Thanks

Response

Response
(active)

To address your clarification questions.

1. For the scenario shown in Double Face MGS W-Beam Placement Options.pdf. As noted previously, there is potential justification for placing the barrier at the outside of the median curve closet to oncoming traffic to shield the majority of errant vehicles. However, there is also the desire, as noted in the RDG to shield the more critical slope, which we noted to be a good general recommendation.

The first option you have shown in this file indicates placement of the barrier on the 10:1 slope side of the median. While this poses little to no issue for oncoming traffic, it may pose a concern for the reverse direction traffic. Reverse direction traffic would tend to exit the steeper slope and become airborne. This may lead to underride or override of the median barrier depending on the width of the median. The trajectories of vehicles traversing off a 6:1 slope onto the 10:1 as shown has not been specifically analyzed, but we do know that the vehicle might be airborne, have the suspension compressed/bottomed out, or be rebounding from the suspension compressing. All of these items would affect vehicle interaction with the guardrail and may degrade vehicle capture in a manner similar to what I noted previously for traversing up the 6:1 slope. The extent of the concerns is difficult to determine without analysis of the ditch geometry and the vehicle trajectory.

In summary, there are potential concerns for vehicle interaction with the median barrier for both directions of the slope traversal that cannot be ruled out without further analysis. In addition, we would tend to agree with the more general recommendation in the RDG to shield the steeper slope, with the caveat that there may be an additional consideration when horizontal curves are installed.

The second page of that attachment shows an option where you are shielding the curve of the road closest to oncoming traffic where impacts are more likely to occur with individual barriers on the outer edge of each curve. There still may be issues with ditch traversal in this instance as discussed previously, but these impacts are less likely due to the roadway curvature. In this case the vehicle impacts are more likely to occur with the barrier placed close to the travel way, which would not pose that concern. Overlapping or fish-scaling of the installation as shown also eliminates the need for the barrier to traverse the ditch. We believe that this is likely the better option.

2. For the scenario shown in Clarify #4 in the Response. For s single roadway curve as shown, there is a reasonable argument for shielding the outside edge of the curved median for oncoming traffic as this is the most likely impact scenario regardless of the slope at that location (assuming that both slopes are traversable as shown). As you noted, for a barrier placed to shield the 6:1 slope, there is a higher risk in this situation of the impacting vehicles traversing the ditch and the 6:1 slope prior to impacting the barrier due to the curvature. This would cause some of the concerns for vehicle capture noted previously. Thus, placement of the barrier to shield the shallower slope in this instance may provide increased safety benefit due to the road curvature.

Let me know if that clarifies things or if you need further discussion.

Thanks

Date March 18, 2019
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Double Faced MGS W-Beam Adjacent a 10H:1V or 6H:1V Slope