Another one that might be local just to my area...standard-size red or yellow coroplast. Stenciled with this message:
STARTING 39 SQ. FT.
Actually, the "SQ. FT." was an afterthought, and it's handwritten in black magic marker below the "39".
I've grabbed about ten of these along FM407 in Lewisville and Flower Mound, roughly between Garden Ridge on the East and Chinn Chapel on the West. Most have been nailed to utility poles, but some were on wooden stakes in the ground.