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I received the attached memo from FHWA today (INFORMATION: Design Considerations for Prevention of Cargo Tank Rollovers - September 3, 2010). I have only skimmed through the document, but I am concerned that FHWA is recommending the use of taller vertical barrier without considering the effect of head slap on smaller vehicles. 

If MwRSF could review view this document and provide comments it would be appreciated.  I wish to send a letter back to FHWA indicating my concerns about the use of taller barrier wall without considering head slap.

I have briefly reviewed the attached information. From my review, FHWA has noted the concern for head slap against taller TL-6 barriers such as a 90-in. configuration tested at TTI years ago. However, this same concern does not appear within the guidance to correspond to TL-5 barriers which have commonly been configured with 42-in. high RC parapets. You have correctly pointed out that 42-in. parapets can also pose risks of head ejection and contact against taller barriers. I believe that Dean has uncovered the risks associated with ejected passengers resulting in serious injuries and/or fatalities after analyzing accident data for the State of Kansas as part of the median barrier study.

I believe that the roadside safety community needs to be careful about blindly placing a large number of tall, rigid barriers in more locations in an effort to contain the rare occurrence of a tractor-tank trailer into piers and other structures, especially if it results in much greater risk of injury/fatality for occupant of passenger vehicles to have partial body/head ejection against tall, rigid barriers. If deemed necessary, it would seem reasonable to utilize TL-5 barrier designs which can both prevent catastrophic crashes as well as reduce/prevent head slap against tall parapets for the occupants of passenger vehicles.

In December of 2021, MwRSF performed a MASH TL-6 full-scale crash test of an optimized, 62-in. tall, single-slope concrete median barrier with a 5.5-degree front batter. This barrier was designed in accordance with LS-DYNA FEA as part of a multi-year USDOT-funded effort through the Mid-America Transportation Center (MATC). The test vehicle impacted and was contained by the barrier, but due to internal sloshing of the water ballast, the trailer subsequently tipped onto its right side and slid for more  than 100 ft before executing another 180 deg roll.

Barrier heightw as one parameter which was suggested to mitigate heavy tank-truck trailer rollovers. However, barrier systems are not responsible for many of the heavy truck rollovers. In many locations, overcorrecting/oversteering may have a more significant effect on trailer instability than impacts with barriers. Further research is warranted to determine if a taller barrier can prevent heavy truck rollover.

Additionally, since the head ejection study was published, two major automotive updates have been implemented into production vehicles. First, side-cushion airbags are now standard in most production vehicles. Side curtain airbags are intended to prevent the occupant's head from extending out of the window by deploying vertically and covering the entire impact side of the car. Secondly, sill heights on production vehicles have increased to improve side-impact crashworthiness, decreasing the risk of occupant partial or full ejection. Both factors have signficantly reduced occupant interaction directly with a barrier or roadside feautre.