I don't do dimensional signs, but shouldn't HDU signs be backed by something that won't snap easily? (aluminum sheet or something of that sort)?
HDU doesn't snap easily. It bends quite a way and then snaps with a surprizingly load bang. Try snapping some over your knee sometime...it takes some effort.
The metal end pieces that held the sign were badly bent indicating a fair amount of force was applied. It would handle wind loads no problem...it just wasn't intended to hold back a person determined to break it.
I doubt aluminum sheet would have saved it.
I usually make two sign faces and bond a 1" square steel tube frame in between them. This sign was fairly small at 4 feet wide and I felt it would be strong enough. I didn't think of the possibility of someone running through it.