I was averaging ten miles per hour behind a semi-truck for about an hour, so I had plenty of time to ponder the "wide turn" sign on the back. I wondered how useful it was, considering the fact that it was pretty much only visible from about ten yards. I wondered if anyone ever missed noticing the truck was slowing down because they were trying to see what the sign on the back said.
A sign that you have to struggle to see and perhaps think about can sometimes be worse than no sign at all. It's great to create visual cues. We put stuff by the stairs that we want to take with us when we go out. That is about 99% mistake-proof for us.
But a wide turn sign should be huge enough to see at a glance while driving or not exist at all. That's my opinion, anyway.