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Could you check out the question forwarded below, hopefully it is a simple one for ya... basically the crux of the question: is it necessary to have clear space above our barrier for any safety or head slap type issues?  Mostly my initial thoughts are that as long as the barrier is poured/formed separately, and the object above doesn't affect the integrity of the barrier, does there need to be clear space above it?  Could aesthetic treatments even be flush with the face of out barrier?

We are hoping to get some clarification from you regarding the need for the 6" lip at the top of the safety barrier adjacent to a wall surface.  Attached is the retaining wall detail we are proposing to use for the west wall on the 70/71 (Project 3) separating the freeway (down low) and the urban avenue (up high).  The current proposal calls for the 6" lip, but that a 2" aesthetic enhancement panel will be placed on the wall above the barrier (labeled on the graphic as the Pilaster Projection).  This basically means that only 4" of exposed lip is available above the wall.

Does the 6" lip have any safety benefit, or is it more just there for constructability to ensure that enough concrete is placed at the top to avoid cracking?  The panel will not actually be sitting on the barrier, thus the barrier top would still be constructed with a 6" lip.

Is this practice acceptable to ODOT?

Previously, MwRSF conducted a study to quantify the extent an occupant's head can extend out the side window of a vehicle during oblique impacts with longitudinal barriers. From this study, we established a "head ejection envelope" which we have used as a template for barrier geometry when designing rigid barriers over 35 inches in height. Keeping all barrier components and attachments out of the head ejection envelope greatly decreases the possibility of head slap during a vehicle impact / redirection. This study was documented in the attached paper (accepted for publication in ASCE " Journal of Transportation). Please note that this ejection envelope was developed for vertical faced barriers and may not directly apply to barriers which induce wheel climb and subsequent vehicle roll away from the barrier.