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My question is about placing a slotted hole in the post flange that is larger than what was used in the crash test. This is shown on the attachment. Thanks.

In general, it is not a good idea to change a critical element of the design from its crash tested variation. An increased hole size in the front flange would reduce the capacity of the steel post to resist bending. Horizontal slots would exaggerate this reduction of post capacity, especially for those located closest to the base plate when moments are maximized. Of course, the overall effect that the slot versus hole has on a barrier system safety performance would depend on how conservative was the design initially. At this point, we have not been provided much information in terms of design/test level, construction details, and actual crash testing results. Further details and information would be needed to further explore this issue.