What is this sign made out of?
https://goo.gl/maps/A3fExnt7tYpbGmms8
I've not seen this in the wild before. If you go forward a few paces and reverse the direction, you'll see the back of the guide sign and the EXIT ONLY appendage that was added later.
I think we call them "slatted guide signs" on these boards, but I'm not sure if they have a more formal or technical name. I think the slats are just aluminum, but I've not been close enough to actually touch one of them, since few of them are mounted closer to the ground (https://flic.kr/p/FLGz3L).
They're scattered around Alabama; a bunch of them are still standing in Mobile and a few in Huntsville, but I'm not sure how many in Birmingham are going to hang around after current construction ends.
Here's the back of one in Huntsville with EXIT ONLY embedded in the slats, on US 72 and AL 255:
(https://live.staticflickr.com/964/27145112337_36a85e9230_b.jpg) (https://flic.kr/p/HmHNbe)
I've also seen them in Delaware.